Optional imports

[manolomartinez]

We should perhaps decide which external libraries are non-optional (numpy, matplotlib, scipy, I think) and which ones are optional. For the optional ones we should do something like:

try:
    import optional_lib
    optional_lib_exists = True
except ModuleNotFoundError:
    optional_lib_exists = False

We would then check for that flag wherever the optional import would be used. I reckon gambit, ternary, and perhaps others should be optional.

[stephenfmann]

pygambit is now an optional import. Trying to run a function that requires it will trigger an error message telling the user to install pygambit version 16.1.0 or above.

[manolomartinez]

So this is now done, right?

[stephenfmann]

For pygambit yes, so this can be closed.