Merge pull request #62 from tayral/fix_matsub_index_to_linear

Various small fixes in imfreq Gfs.
gf: correct bug in matsubara_freq mesh point

- bool at_end() for a mesh point means AFTER the last point ,
as end in STL. It was not correct ( +1 missing).
- also the at_end was not computed in the mesh product.
- it slightly changes the test_fit_tail --> just changed the output.
--> did this bug affect other functions/codes ?