Feature slice evaluations and first test files for pytest

In the domain class functions for slice evaluation in the 12, 13, and 23 planes were added, respectively. Two test files that can be called with pytest have been added, testing 1d projectors (convergence rates) and spline space 1d (plots can be generated). The Greville points are now part of the spline space class. TODO: the multi-dim global projectors should be built using the working 1d projector class.