feat: added support for downloaded or old force fields

[PabloNA97]

Newest AMBER force fields are saved as leaprc. files in the $AMBERHOME/dat/leap/cmd directory. To use them, it is simply enough to provide the biobb with the name of the force field file you want to use (ff_name) and then find the corresponding leaprc.ff_name file.

However one may want to use one of the older force fields contained in the ./oldff directory inside the previous folder or simply download another leaprc file and provide the path directly.

Therefore I think this new implementation is more general and would permit the use of additional force fields through the biobb :)

[adamhospital]

Very nice!
Before merging though we need to replicate this new functionality to the rest of building blocks that have the same forcefield property (add_ions, solvate, linear_structure)
We will take a look.
Thanks!