Refactor pyxmipp3/core to reuse xmippCore/core and eliminate duplicated sources

[albertmena]

The most of the content of xmippCore/core is duplicated in the repository pyxmipp3/core (not the CMakeList.txt). Any change in core should be replicated in the repo. In the pyxmipp3 repo point that folder to the xmippCore/core one, avoiding that duplication.
Also is possible to explore other solutions

[oierlauzi]

Closed in #7