fix license problems in GSA

Some files in the matlab/gsa subdirectory have copyright status incompatible with inclusion in Dynare. At least:

fdjac.m LPTAU.m optget.m Sampling_Function_2.m

Other files do not have any copyright notice, their status should be checked.

The copyright status of the GSA manual is not explicited.

Once these issues are fixed, the license.txt file should be updated accordingly.