Remove default c++14 flags on clang 3.4 ...

- a flag left by mistake after checking portability on C++1y...
mc_tools: simplify measure_aux

- for pieces that need to be precomputed for several
measures.
- put them under shared_ptr, and register then with add_measure_aux.
- they must be callable, as void ().
- TODO : add this in the doc when tested
gfs: mesh product, rename shape method...

- just a renaming for readibility.
arrays: correction of value_type of matrix_expr

- forgot to correct the value_type of matrix_expr, and vector_expr
as was done long ago for arrays...
- also added cases for arrays until dim 10
- TODO : replace this trait in arrays with a tuple tools for any dim..
  not urgent.
gfs, matsubara: correct unary - for matsubara_freq

- The unary - of matsubara_freq was not correct ...
missing () around the ==
remove warnings in compilation

- triqs & tests now compiles with -Wall on clang (3.2)
Checked on OS X.
Todo : recheck on other compilers.
arrays: add gelss
gf: new tail fitting
arrays: to_python now raise exception when array is empty

- otherwise it was simply crashing .
- now raise a C++ exception
- adapted cython wrapper to catch it as a python exception
Merge remote-tracking branch 'triqs_lib/master'

Conflicts:
	test/triqs/gfs/gf_retw.cpp