Found, not found, and incomplete programs

[dgasmith]

As was mentioned in #212, some programs require additional dependancies to function beyond the executable or Python project. Two examples are Orca (CCLib) and NWChem (networkx). The CLI support for this is fairly straightforward as shown in #212 by @loriab:

>>> Program information
Available programs:
dftd3 v3.2.1
mp2d v1.1

Incomplete programs (see devtools/conda-envs for install help):
nwchem -- needs networkx
orca -- needs cclib

Other supported programs:
cfour entos gamess molpro mopac nwchem openmm psi4 qchem rdkit terachem torchani turbomole

The main question is if we need to expand the found() syntax or some other resource to determine the difference between a "runnable" state and "found, but incomplete". A few proposals:

• Leave found() as is and have a new function runnable() that includes dependancies in its checks.
• Add an additional kwarg to found(include_dependancies=True).
• Have found() return an enum or similar object that contains a variety of states True, found_executable, found_dependancies, None.