-> 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.
- little details : code cleaning, clang formatting, along
with documentation writing for c++ gf.
- separated the mesh in small class for better doc.
- work on documentation : reorganize specialisation, ...
Now the tail have a fixed size. It actually makes everything simpler. I took
order_min = -1 and order_max = 8. This makes the tails compatible with the
previous implementation. However we might want to change this to something like
-10, 10 so that they are self-contained. This commit should also fix issue #11.