Extraction tab contains error input widgets that take error in the
+/-, so as the half-width of the interval centered on the value.

A new kind of control was produced that inlines the text of labels
with the input box. Another variant places two labels around the input
box (suitable for units).

As much as possible of the code uses MSnbase objects to keep data. For
example, instead of CSV files, the chromatograms are now inside a
Chromatograms object.

Things have been adapted to this until the preproc. The preproc stage
is still not working properly for MS2 part.

Retention time functionality in chromatograms.

Some naming changes.

R/extraction.R (gen_rt_range): Generates rt range matrix given rt-s
and tolerance.

The problem seems to have been with the ordering in the
verif_prec_fine procedure.

More to follow.

But, MS2 are way off

Also, move `...prescreen_df' to extraction.R and rename it to
extr_rmb. Create driver level `extract' function that will be used to
call the backend extraction function based on user's choice.