To install, you also need the difxcalc11 source code, and have numpy
installed (as well as pytest for running tests).
Assuming you are at the top of the directory of pycalc11, you can do the
following:
git svn clone https://svn.atnf.csiro.au/difx/applications/difxcalc11/trunk difxcalc11 pip install -e .
If you already have the difxcalc11 source and data directories somewhere,
either create a symbolic link to its location, or set the environment variable
DIFXCALC11 with the path to it.