meqtrees: Simplify package detection

Instead of hard-coding the Python path for additional MeqTrees packages
we now import the package as a last resort if it is not found in the
default locations and query the module path instead.
purr: Provide alternatives for 'cp -u' and 'mv -u'

On Linux platforms the cp and mv commands accept a -u parameter to
update the destination only if the source is newer. This flag does not
exist in the Mac versions but the Mac has a capable alternative in the
form of rsync.
purr: Escape spaces in pychart histogram filenames