Improve monomer adding-swapping conditions

[nchenche]

When clicking on "+" icon from the monomer library, a monomer is automatically added at the C-ter of an existing sequence. Some conditions should be checked when adding a monomer to an existing sequence:

No addition is possible if:

• C-ter sequence is capped (id est, no rgroup is available to be linked to another one)

No swapping is possible if:

• swapped monomer is N-ter/C-ter and its n-ter/c-ter rgroup is not free (id est linked to another rgroup from a non peptide bond)