Put main package in src directory

[steven-murray]

The problem of it loading the local package rather than the installed one is a common problem and the most suggested fix is to put your actual package in a (non-python-package) src/ folder. So you'd have:

corrcal/
    setup.py
    src/
        corrcal/
            __init__.py

This will also help in testing (because pytest will by default pick up the local package rather than the installed one, which is usually not what you want!)

See https://blog.ionelmc.ro/2014/05/25/python-packaging/ for example.

[piyanatk]

PR submitted #30