Merge pull request #1671 from alaingdl/master

improved version for GAUSSFIT() + initial import of "test_gaussfit.pro" + managing X steps not equal to unity + better method to estimate 4 of the 6 parameters (seems to be enough to converge !)

for the record : 
-- few keywords are not tested or are not OK
-- irregular steps or gaps in X are not tested (might works for small gaps & irregulars steps)