Added method solveSystemLeastSquares().
Added support for tensor product topologies to the following methods in CamelliaCellTools: permutationMatchingOrder, subcellOrdinalMap, mapToReferenceSubcell.
Fixed a couple bugs in CellTopology.
Added support for a RefinementPattern on a Node (allows consistent logic with tensor product topologies; maybe only useful for tests).
Added support for 0-dimensional topology to MeshTopology.  Not practically useful, but makes the implementation of the Node topology refinement pattern cleaner...
Restored commented-out legacy code that ensures a certain ordering in RefinementPattern's childrenForSides for 2D space-only topologies.
Added several tests, including several for space-time topologies and refinements.  All these pass.