gf: correct auto_assign

- g(om_)(i_,j_) was not working.
- add the triqs_clef_auto_assign function for matrix_proxy.
- simplified a bit the triqs_clef_auto_assign for gf.
arrays: add lazy call to matrix_tensor_proxy

- correction to previous patch
clef : correction of TRIQS_CLEF_IMPLEMENT_LAZY_METHOD for clang 3.3

- there was a compilation pb with clang 3.3
- replace the decltype (with this !) by an explicit type
correction: previous commit, forgot a namespace

- in some case, need to add triqs::clef in the macros.
(was not compiling doc).
arrays: fix fold and update documentation

- fold was not correct in e.g. passing an int as init instead of a
  double (was leading to narrowing in return).
- better return type deduction.
- there was an error in the doc (order of argument in the lambda !)
- add a more complex example (Frobenius norm of matrices).
Added ising2d test for statistics.
Statistical tools documentation.
Added constructor of observable on binned_series
Made autocorrtime calculation from binning faster.

Bin size is doubled at each step to converge faster and reuse previous binned series.
statistics : fix get_value_type detection

- clean a bit the auto_correlation_time
added a missing dependency for compiling doc
exceptions handling : slight improve

- separate the stack trace from the error message for better python
  handling
- separate triqs::excption from triqs::runtime_error
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.