arrays: add make_unit_matrix and norm2, norm2_sqr
clef: correction of macro

- a strange issue, why was it not working ?
- anyway, the separation is useless (was a try).
c14, tuples : various fixes.

- Various details (for_each_enumerate impl)
- also put all tuple stuff in tuple_tools.hpp
- add std14 namespace for usage of c++14 helper types
gf: add C++14 impl of meshes products

-  add c14 include
- the C++14 is lot more readable (due to generic lambda).
- for mesh/product.hpp -> now 2 versions (C++14 and C++11 for temporary
  backward compatibility).
lattice: clean old classes, add bz_mesh, gf on bz
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
Added basic doc + additional test for tail fitting

- added test for a 'real-life' GF + corresponding output
- added basic usage documentation for tail fitting from c++. Full implementation details yet to be written
Fixed fit_tail for pos. and neg. matsub + bosonic

-> code was previously assuming mesh with only positive, fermionic matsubara freqs
-> changed wn_min to n_min (was misleading, since it was an index, not a frequency) / same for <-> max
-> changed doc accordingly
Extending fit_tail for scalar_valued (+ test)

-> extension by using reinterpret_scalar_valued_as_matrix_valued
Fixing test outputs.
Made test gf_notail clearer. Still pb at line 52