New feature: group and summarised Spectra

[lgatto]

The filterPrecursorMaxIntensity() (see #290) conceptually works as follows:

1. Group spectra based on a given criterion, here those that have the same precursor MZ,
2. Group these or filter one of them, here the one with the highest precursor intensity.

This could be generalised with groupSpectra() and summarizedSpectra() functions. But before embarking on something like this, we should rework the documentation (see #288) to clarify which functions would fall under this paradigm.