SOAP could be packaged in a more standard way

[jchelly]

This was provoked by some problems with module search paths while trying to run the halo matching script, which uses parts of SOAP.

Currently SOAP is just a bunch of source files in a directory and to run it you need to be in that directory or else you get import failures. We could turn it into a python package so that it could be pip installed (including dependencies) and we would no longer be relying on the current working directory to ensure all the modules are found.

We could also modify it to run with something like "python -m SOAP" instead of requiring the path to the main program source file.