Define energy bin as min <= energy < max.

The energy bin is now defines as min <= energy < max, hence the maximum bin energy is not included in the bin.
Add GPha.hpp file.
Initial version of Xspec spectrum class GPha.

The GPha class handles spectra in Xspec format.
Initial version for GPha.cpp compilation.
Add an xspec module.
Add xspec module unit tests.
Exclude xspec module unit test executable.