LALInference has undeclared dependency on cpnest

The lalinference_cpnest script has a top-level import of cpnest, which isn't available on any of our reference platforms*. We should do one of the following

• remove lalinference_cpnest from the main python*-lalinference package (possible supporting it in a standalone package that can depend upon cpnest in another way (pypi?))
• package up cpnest for all of the reference platforms
• wrap the import cpnest calls in try/except to give a better error message explaining that this executable relies upon an 'optional' extra.

*a conda package has been proposed here

/cc @john-veitch