slightly more useful issue templates

refs #110

.github/ISSUE_TEMPLATE/bug.md

+---
+name: Crash / error / performance problem report
+about: Something that should work but does not, works very slowly, or produces wrong results.
+---
+
+## Minimal code example to reproduce the problem
+
+> Include a reproducible sample of code that causes the problem. Others should
+> be able to reproduce your problem by just copypasting your code into Julia
+> shell. Use `download` to get any necessary datasets from the internet.
+> Preferably, keep the sample code as minimalistic as possible.
+
+```
+using COBREXA
+download("https://badmodels.org/model1.xml", "model.xml")
+load_model("model.xml")
+# ...
+```
+
+## Expected result
+
+> Describe what you expected to happen if the above code worked correctly, and
+> why you think that behavior is correct.
+>
+> If possible, copypaste the expected result directly to the issue description.
+
+## Actual behavior
+
+> What was the unexpected thing that happened instead?
+>
+> If possible, copypaste the error message or the wrong result directly into
+> the issue.
+>
+> In case you are reporting a performance problem, include some timing
+> information, using e.g. `@time` or `@benchmark`.
+
+## Optional: Suggestions for fixing
+
+> Feel free to suggest whatever change in the package that might remove the
+> described problematic behavior. If you already have a fix, you may want to
+> open a pull request instead of the issue.
+>
+> Erase the section if the cause of the problem is unclear.
+
+## Optional: Environment
+
+> If relevant to the problem, it is beneficial to provide all information about
+> your environment, including the version of Julia. If possible, paste your
+> package status here (type `]status` into Julia shell).

.github/ISSUE_TEMPLATE/custom.md

----
-name: General issue (performance or bug)
-about: Describe your issue related to performance or a bug
-title: ''
-labels: ''
-assignees: ''
-
----
-
-**Summary**
-
-**Environment**
-
-*Please provide all information on your environment here, including the
-version of Julia*
-
-**Steps to reproduce**
-
-    1.
-    2.
-    3.
-
-**Expected behavior**
-
-**Actual behavior**
-
-**Additional information**

.github/ISSUE_TEMPLATE/enhancement.md

+---
+name: Feature request
+about: Suggest new functionality to be implemented in the package
+---
+
+## Target functionality
+
+> Describe the functionality that you would like implemented. Link possible
+> implementations or methodological descriptions, and datasets that can be used
+> to test the new functionality.
+>
+> Explain how the enhancement combines with the rest of the package (e.g., if
+> it is complementary to some existing features), and possibly refer to a
+> use-case that is going to be benefited by the implementation.
+
+## Desired output
+
+> Write a (pseudo)code example that explains how you would like the feature to
+> work.  Suggest what would be a desirable output (e.g., type, data layout,
+> ...) of your pseudocode.
+
+## Optional: Suggestions for implementation
+
+> Feel free to point out viable ways to add the functionality. If there are
+> more implementation alternatives, feel free to point out advantages and
+> disadvantages.
+>
+> If you have a code that implements the feature (at least partially or with
+> problems), you may want to share it by opening a pull request instead -- it
+> is easier to work on and enhance an existing code piece than starting from
+> scratch.

.github/ISSUE_TEMPLATE/feature_request.md

----
-name: Feature request
-about: Suggest an idea for this project
-title: ''
-labels: ''
-assignees: ''
-
----
-
-**Is your feature request related to a problem? Please describe.**
-A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
-
-**Describe the solution you'd like**
-A clear and concise description of what you want to happen.
-
-**Describe alternatives you've considered**
-A clear and concise description of any alternative solutions or features you've considered.
-
-**Additional context**
-Add any other context or screenshots about the feature request here.