mc_tools : add precomputation (first draft)

- add precomputation as auxiliary measure,
computed before the accumulation of measures.
add many body operators in python (old version)

- The old code, which was in the solvers,
is used in several apps (dft, ctqmc_hyb).
It is really a lib, even though it is now superseded
by Igor and Andrey's library...
(to be deprecated at some point).
add little modif to make the dft apps
Merge remote-tracking branch 'triqs_lib/master'
test/triqs/gf: test of on_mesh() added