[GFS] overwrite all entitysets in the gfs tree

Currently, the grid function spaces are built bottom up, e.g. first the leaf spaces are constructed and passed to the other nodes and the root of the gfs tree. By this process, nodes in the gfs tree might have different entity sets which are then not setup correctly in the gfs construction process.

This patch overwrites the entity sets of all child spaces of the root with the entity set of the "leftmost" child prior to creating the ordering tree. This fixes issues where an entity set was not setup correctly when the ordering was created, as now all leafs share the same set. However, with this patch, the entity set object can not be provided by the user anymore, as it might be overwritten internally.

We adapted the tests and added two new tests where the former ordering creation failed (testpowergridfunctionspace-fixedsize.cc and testplasticitygfs.cc). The better way to fix this might be to create the gfs tree top down instead of bottom up and passing the entity set directly down to the leafs from the root tree. This fixes #62 (closed) and #70 (closed) .