LoadMcStasEventNexus

This pull request adds 7 new files:

1.	CMakeList.txt
2.	LoadMcStasEventNexus.h
3.	LoadMcStasNexus.h
4.	LoadMcStasEventNexus.cpp
5.	LoadMcStasNexus.cpp
6.	LoadMcStasEventNexusTest.h
7.	mcstas_event.h5

By mistake LoadMcStasNexus.cpp and LoadMcStasNexus.h was added  to this pull request. They should bed ignored when merging with mantid/master