[FEATURE] Assign peaks to database

[AlanRace]

Allow matching of accurate mass (with some tolerance) of a selected ion image (or peak) to database(s).

• The parser must define which databases are appropriate for the style of data loaded
• Must implement Database class which handles downloading of database (if necessary) as well as performing a query
• Must allow selection of a tolerance (e.g. ppm) for narrowing down the results
• Optionally allow matching against adducts and isotopes for improved confidence in assignment (scoring)

[AlanRace]

From discussions with Kenny Robinson:

• Deisotoping peaks for the assignments list
• Isotope ratio to inform assignments
• Ability to add in custom adducts, e.g Ag or Cs.
• Ability to add in custom chemical formulae, e.g. deuterated compounds etc
• Preview of peak for quality checking
• Ion image preview for peak
• Filter your assignment list by column, e.g. filtering out kingdom or by pathway