clef : add apply_on_each_leaf

- apply_on_each_leaf: apply a function object to every leaf of the tree
- add some macros...
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