Renaming data

[lgatto]

Following up from #14, I am wondering if we shouldn't also rename the data? It was obvious that modifications referred to the Unimod modifications when the package was called unimod, but if it expands in scope, it might be preferable to anticipate further changes and rename the data to unimod.

I think aminoacids and elements are probably still fine.

Ping @sgibb

[Arthfael]

It may be a good idea to have some aliases for internal compatibility? I just found out about the breaking changes from unimod to PTMods and whilst I can and will make the necessary transition... it happens at a rather inopportune time...