gradient of a dolfin expression -- testint purposes only
helpers-- ADDolfinExpression; for calculating the diff of flow
tidy
remove test.py
swp and dat
change n turbines
Changed the _is_downstream method so that Heaviside functions are used instead of if statements -- this was a suggestion by Matt to speed up the AD. Not fully tested yet.

Also fixed an error in the Larsen individual factor.
added an example