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This rationalise the way we report errors from libsbml in a few different
places.

Fixes #100

- standalone literals return Num type
- constants are now converted properly

fixes #102

@exaexa is there a place we can view all the names and add more? we are probably missing a bunch

...to make the work with custom datatypes easier.

Also:

* replace `@warn` with `@error`
* actually test the logging message in `readSBML`, instead of showing an extra
  message saying to ignore it.

This potentially saves a lot of headache with various custom functions or stuff
where support may be complicated/ambiguous. In this precise case, we're
allowing it because the user needs to `@register` the factorial function with
Symbolics manually, so that it can be translated properly.

This got solved quickly, thanks Mose!

:'(

Paul reported in #64 that empty parameter list actually causes this to break
because the generated Any can't downcast to String automagically, which should
be fixed.

Closes #54

...to allow distinguishing stuff like Euler's number from variable name `e`