Merge pull request #137 from ComputationalRadiationPhysics/dev

Release 1.2.1: Non-Extensible Data Sets
Merge pull request #139 from ComputationalRadiationPhysics/dev

Release 1.2.2: Version Number
Bump Version Number to 1.2.3
Write 1.2.3 Change Log
Merge pull request #145 from ComputationalRadiationPhysics/release-1.2.3

Release 1.2.3
Merge pull request #146 from ComputationalRadiationPhysics/master

Merge changes from master back to dev branch