improve handling of empty spectra

[cpauvert]

Consider adding a remove_empty() function to clean up the spectra, the peaks and the metadata files from the object returned by process_spectra().

Could be coupled with check_spectra() to flag the elements of the list.

[cpauvert]

remove_spectra() could be a generic function

• for raw spectra: "just" subset the list
• for processed spectra: subset the processed spectra list, the peaks list and the metadata table

could use is_a_processed_spectra_list.R