Fix extruded mesh "off by one" layer construction

The constructor to ExtrudedMesh now considers the layers argument to
count the number of cell layers (1 or more).  We add error checking of
the argument and then immediately translate to PyOP2 style
counting (repetitions of the base set in the extruded set).  Update all
tests to follow this change.
Correctly default layer_height if not provided

Previously even though we had code to add a default layer height it was
never reached because we errored beforehand if no layer height was
provided.  Switch this check to ensure we pass either a kernel or a
preset extrusion type, and update docstring appropriately.