Fixing conj keyword for GULP (#211)

[DBodesheim]

See #211

[DBodesheim]

Consider the following example script:

import stko
import stk
from pathlib import Path

gulp_path = Path('/path/to/gulp')

gulp_opt = stko.GulpUFFOptimizer(
    gulp_path=gulp_path,
    output_dir="test",
    conjugate_gradient=True,
)

mol = stk.BuildingBlock(smiles='Cc1ccccc1')
gulp_opt.assign_FF(mol=mol)
gulp_cage = gulp_opt.optimize(mol=mol)

With the old implementation we get the following error:

With the new one, the unit keyword is not present anymore and the optimization runs through (see attached files).

min_example_gulpchange.zip

[andrewtarzia]

For backwards compatability, is it possible to "easily" determine the gulp version and then use conj unit for <6.4 and conj for >=6.4?

Even if it is "wrong", which I am unsure if it is based on the output files I have found and I have not analyses it closely enough, this solution means that the wrong solution from before is not broken in this version. What do you think?