Moved a comment to opencl_handler.hpp.
Discontinuous spaces can now be forced to include only functions
with supports lying only on elements belonging to a segment.
Enhanced documentation of some Space classes.
Added a Python example demonstrating solution of a problem with mixed BCs.
Raviart-Thomas spaces can now be defined on grid segments.
Added strictlyOnSegment argument to the constructor of the R-T space.
Corrected a duplicate test of the R-T space.
Added sctrictlyOnSegment to the constructor of PiecewiseLinearContinuousScalarSpace.
Cleaned leftover comments.
Extracted identical code chunks used to init. flat local -> local dof maps
in various spaces into an external utility function.
Commented the mixed_bcs.py example better.