Update README.md

Improve demo python code.
Update defode/functions.py

Add definition of __all__ and define pow.
add missing import
Create .gitattributes

Enforce line endings.
Documentation and minor code tweaks.
Render variables in sort order by default.

This means that variables in the same compartment come out close
together, which may be nice for memory locality.
Pull out time variable from list of time-dependent variables.

Remove the master time variable from the list of time-dependent
variables at source, to simplify downstream code.
Add in factory function for compartments.
Add in 'pow' method.