test/triqs/mc_tools directory added, with an example of simple random walker
det_manip: try_insert_from_function(i,j,fx,fy,ksi)
doc: det_manip, link towards the cookbook added, explanations added
Work on gf

gf: security in the case beta<0 added in the mesh construction
gf: inline added in slice
test/triqs/gf: test of on_mesh() added
gfs: scalar for two-real_times
test/triqs/gf/ renamed in gfs, test gf_retw.cpp completed
gfs: evaluator homogeneised
two_times: evaluator corrected
test/triqs/gf/ renamed in gfs, test gf_retw.cpp completed

+ Correction after rebase

Fix a test : gf_re_im_freq_time
There is an issue with the last point.
To be fixed.
gf : cleaning the imtime evaluator