clef: correct TRIQS_CLEF_IMPLEMENT_LAZY_METHOD

- The name of the object was left to the name used in the test/debug.
correct include in ising2d test.

triqs include goes FIRST.
(for C++14 mode in clang, e.g.).
Added linear_to_index to matsub_freq
Exposed MeshImFreq. Added field start_at_0 in ALL cases in hdf_write of mesh matsubara_freq. Otherwise, not same interface for case positive_only/not positive_only.
Added accessor positive only to MeshImFreq
Remove normalization from Hilbert trafo
Small fix in gf doc.