pyInterface: start cleaning up pyInterface/package.

Start cleaning up the rootpwa python package by moving quite a number of files
dealing with amplitude calculation to a newly created legacy script directory.
There is no replacement at the moment, but this will be added over the course
of the next few commits.
pyInterface: Add new amplitude submodule.

Add a new amplitude submodule to the python package which only contains one
function at the moment: it calculates the amplitude for an input file and a
keyfile and saves that to an output file. With this, it should be possible to
write a new calcAmplitudes.py quite comfortably.
pyInterface: Clean up the python package some more.

Move more files to the legacyScript directory and clean up some of the
__init__.py files. Also change the core submodule from a directory to a .py
file.