Force field selection

[alberto99]

We now have a limited set of amber force fields to load, and load only one.
We should be able to load several. E.g. ff14SBside + parmbsc1 + GAFF

The ordering in which the force fields are introduced in leap is important -- will this be a list that we load in order into leap or will meld have some logic? I would favor this being responsibility of the user.

We should also have the option of amber default names, not only dictionary names. MELD should first look in the standard AMBER directories for the name of the force field ordered, if not found, try to match with a dictionary as we do now. This way, users can access future force fields without having to change meld every time.

[jlmaccal]

One possible way to fix this is to use leap_header_cmds. We could make it so that setting the force field to None skips loading anything automatically.

[alberto99]

I think that should work. so basically setting force field to none should not execute
leap_cmds.extend(self._generate_leap_header())
and the string which loads mbondi, and force fields is the responsibility of the user to incorporate into leap_header_cmds.

[GoldEagle93]

I will try to look into this in the coming weeks.