Fix crazy compiled Expression syntax

[Prof Garth Wells]

Code like

 f1 = Expression("a*sin(3.0*x[0])*sin(3.0*x[1])*sin(3.0*x[2])", degree=2, a=1., name="f1")

(from a unit test) mixes arguments that a parameters in the code string, e.g. a, and arguments that affect the Expression type, e.g. name and degree.

There should be clear separation between arguments that appear in the code string and other arguments, e.g.

 f1 = Expression("a*sin(3.0*x[0])*sin(3.0*x[1])*sin(3.0*x[2])", arg=[(a, 1.0)] degree=2, name="f1")

[Jan Blechta]

Or arg={'a': 1.0}.