Improving Python API cmake

[Lestropie]

Some issues / undesirable behaviours I've encountered in trying to create an external project containing a Python command.

• Creation of softlinks to the Python API and command files from build directory to source directory should I think default of OFF rather than ON. It is non-standard behaviour, and causes problems when building a Docker container where absolute filesystem paths may differ between intermediate build and final images.

• The task of creating the Python version file appears to not be being registered as a dependency for the API; but the API crashes out immediately in its absence.

• I think currently the entire set of C++ commands are being added as dependencies for all Python commands, on the premise that if it is not specified within cmake files which commands are required for a specific Python command, the only safe thing to do is to build all of them. That set of dependencies may however be encoded elsewhere. For instance, where I seek to distribute a particular MRtrix3 extension via a container, I can encode within the Dockerfile which subset of C++ commands needs to be built.