v0.6.0

Changes:

• The main change in this release is a very heavy refactoring of files. Functionality wise, the library hasn't changed that much, but every model has been split into it's own file.
• Updated the info text for the root class PVSystems with the contents of the README.md file.

Additions:

• Added battery model together with a validation example model.
• Added User's Guide package with References, Release Notes, Contact and License information.