Update release/fabfile.py.
Re-add some functions and a docstring from master.
Add call to expand in add & sub, fix linear_expand.

Call expand early to identify cases where the expression reduces to zero.
Re-add galgebra tests from master.

A number of these tests are now failing, due to the vast changes
in the interface of many of the base classes inside galgebra. They
are added with @XFAIL and should be investigated individually.
Various docstring fixes to please Sphinx.
Update the Sphinx documentation for galgebra.