gammaALPs with fixed GMF model and improved documentation

This release includes two major pull request:

• PR #37 fixes several bugs in GMF model of Jansson and Farrar. It also corrects the behavior of the energy setter functions, so that the redshift and doppler boost are now correctly taken into account. Also, the requirement on the ebltable package has been updated.
• PR #38 improves the documentation w.r.t. to the used coordinate system.

Furthermore, the documentation has been fixed.