Things like conversion between some controls and the yaml config file
etc ...

Prior to this commit, cofiguration consisted of static values and
dynamic variables calculated based on the static values. Changing a
static value meant that those variables had to be recalculated. In a
reactive environment where things change a lot, there was danger that
some parts do not get calculated, or that recalculation leads to
overwriting of other variables. Now, all the dynamic config entries,
such as paths within the project tree are delegated to "accessor"
methods that evaluate their value at the right time.

* R/mix(get_fn_comp,get_fn_extr,get_fn_summ): New functions to
evaluate paths to the comprehensive table, extraction file and the
summary file.

* R/api.R: Corresponding changes in here.

* NAMESPACE: Some updates.

* R/shiny-ui-base.R(rt_input,mz_input): Parameter pref now determines
the label of the control.

* inst/rmd/app.Rmd: Shinyscreen GUI.

* NAMESPACE: Update exports

* R/api.R(create_plots,save_plots,report): User-level functions to
generate and save plots, as well as generate a report containing them.

* R/mix.R: Supporting functions for above.

* R/resources.R: Supporting params for above.

* R/api.R(subset_summary): New function. This will subset the summary
table according to the user specification.

(order_spectra): Updated docs.

* R/resources.R (DEF_ORDER_SPECTRA): Summary table should be ordered
by sets first and then the quality and intensity as criteria.

* R/api.R (extr_data): An unecessary set column was removed from the
m$extr$ms table to avoid possible confusion. Extraction deals with
unique masses, so this column listed only the first set (in comp
table) that contained a particular mass. Later, during the prescreen
stage the set is coming back directly from the comp table.

* R/api.R(prescreen): Split the qa$ms table into three: one to hold
ms1 spectra (ms1_spec), one for ms2 spectra (ms2_spec) and a summary
table (former ftable, now summ). This was done due to mismatch in the
number of entries between MS1 and MS2 spectra and in order to keep
summ table lightweight (no complex objects such as other tables).

* R/resources.R(MS2_SPEC_COLS, MS1_SPEC_COLS, DEF_ORDER_SPECTRA):
Constants COLS define the columns of ms?_spec and DEF_ORDER_SPECTRA
lost set as uninteresting column.