filterPrecursorMz with multiple precursor m/z values

[jorainer]

At present filterPrecursorMz expects a range of m/z values and keeps all spectra with a precursor m/z within that range. I'm wondering if it would not also be nice to have the possibility to filter spectra matching any of provided precursor m/z (see also on https://github.com/rformassspectrometry/SpectraQL/issues/1). So, what I would suggest is the following:

• new filterPrecursorMzRange method (that symlinks at present to the filterPrecursorMz function) which accepts a numeric(2) with lower and upper m/z range.
• new filterPrecursorMzValue method that has parameters mz, ppm and tolerance and keeps all spectra that match any of the mz (given ppm and tolerance).

This is somewhat similar to the filterMzRange and filterMzValue methods we have already implemented. Any thoughts on that @sgibb and @lgatto ?