Add utility for checking / activating phenix and CCP4

[dennisbrookner]

It would be nice to provide some sort of functionality for activating phenix and CCP4. The simplest thing to do (and something I definitely know how to do!) would be to provide a utility like matchmaps.isphenixactive which just checks if phenix is active and prints out the result.

A more sophisticated option would be to allow a user to "save" paths to phenix and CCP4 as environment variables, such that they could be activated in advance some matchmaps.setup function. I think I know how to do this, but I don't know how long the environment variables would last. Are these being added to .bashrcs? Do I have to write code for multiple operating systems?

Finally, the best thing would be to not only save these paths, but to source phenix and CCP4 on the fly within a call to matchmaps. I tend to believe that this is not possible, because every call to subprocess is its own thing that doesn't know about previous calls? But maybe it could be done?

[dennisbrookner]

All of the utilities now check for phenix/ccp4 in the environment first thing and throw an informative error if something isn't active. For now, this is a significant improvement. This will make it into the next release, presumably 0.4.0