Ncrystal

* Add support for python3 (github issue 6).

* Correct version numbers.

* NCrystal_sample.comp (McStas interface): Fix link flags on OSX
(github issue 3) and compilation warnings (github issue 4).

* Fix cross-section and material composition bugs in Geant4
interface. Only affects Geant4 users.
* CMakeLists.txt: Always use absolute rpath (needed on OSX).
* Fix gcc7 compilation.

* Proper behaviour of python interface if Numpy is absent.

* First github based public release of NCrystal.