Adding a module (i.signature.list) to list the signatures

[spareeth]

Now there is no way to see/list the signatures developed for classification. It would be helpful to have a module to list the signatures, or add the functionality to g.list. Also to copy the signature from one group to another !!

g.list sig
i.signature.list -l

[lucadelu]

In 72434:

i.signature.list: added module, see #3526

[lucadelu]

In 72454:

i.signature.copy: first version, for sajid birthday, see #3526