Added the function and test for it.

Refs #7756
Updated parameter default values and names.

Refs #7756
Fixed the failing unit test.

Refs #7756
Merge remote-tracking branch 'origin/feature/7756_muon_skt_times_gausdecay'

Conflicts:
	Code/Mantid/Framework/CurveFitting/CMakeLists.txt
Re #7756