move common Mappings code into new sfepy/discrete/common/mappings.py

- move PhysicalQPs, get_physical_qps(), get_mapping_data(), get_jacobian(),
  get_normals()
- replace create_mapping() by new Mapping.from_args()
- rename original Mapping -> FEMapping
  - move .get_physical_qps()
- update VolumeMapping, SurfaceMapping parent class