Add an insertCompound function

[jorainer]

Add an insertCompound function that allows adding compounds to an (existing) CompDb. The function should:

• alter the database table if provided data contains columns that are not yet in the database.
• ensure column "compound_id" is present and not empty.
• check if a compound with the same ID is already in the database table and show a warning if so.
• invisibly returns the compound IDs.

The function should be similar to insertSpectra, i.e. have a parameter columns to allow selection of specific columns to insert only.

Pinging @mar-garcia

[jorainer]

Add also a deleteCompound function and add a section to the vignette describing all.