This commit fixes an error, at least I think that, in the Python interface of Akantu.

The problem is that several parts needs the ElementTypeMapArray type, such as the mesh, the FEEngine and the material, probably even more, but grep is not able to find them (because of the use of auto).
This type is indirectly registered by the register_material() function in the py_material.cc file.
However that function is only run if the solid mechanics part is enabled.
Since this is almost always the case, it was probably not an issue till now.
However since I have deactivated it, for my development of the structural mechanics part, I run into that issue, there is still one, but I have not found it, I am not even sure if _this_ is an error.

The fix is that the ElementTypeMapArray type is now registered by the register_mesh() function, which is the first occurrence of that type (as far as I can tell) and is always built.

An important side note is, that the InternalField is sill registered by the material.