Fix #52 : Correct 6022015

- forgot to correct the call to self.kind (now returning a char, not an
  int), in previous commit.
Correction of last correction....

The construction from c++ should *not* be changed...
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.
Fixed computation of density for full matsub mesh

-> Previously, calculation was implicitly assuming a mesh with only positive matsubara frequencies.
-> Added corresponding test
-> Also added test of density calculation with or without correct tail.
Changed format of requirement warning in doc

- now more visible
- also added link to applications page