log normal distrib
relation test replaced by 0.5 ulps tests in normstat
polishing documentations of lognormal functors
bad include in logninv.hpp
fixing logninv test
incorrect value in logninv test
Merge pull request #877 from jtlap/logn_statistics

Lognormal statistics
use auto in place of BOOST_AUTO_DECLTYPE when we're always using the auto->decltype form
fix specialization of hierarchy_of for hierarchies, solves problems with policies
tag are hierarchies, try to fix all_reduce
correction fixing #878 about pow_abs
Merge pull request #879 from jtlap/pow_abs

correction fixing #878 about pow_abs
Revert "tag are hierarchies, try to fix all_reduce"

This reverts commit b12acab861e3cf3841d019d56025c67f3cef3ad0.
change hierarchy_of of tags, too intrusive
Introduce is_safe free function.

is_safe was a member function on some of NT2 sequence like
types that made difficult the use of alternate types from
3rd party as such a component.

Contents of this commit :
 - factorize is_safe as a free function
 - remove ::is_safe from the interface of container/buffer types
 - Amend test and usage
raw() is now data()

Still in the process of alignign our API with std containers one,
raw() now becomes data() to follwo C++11 API style. Similarly, the
free function raw() is also renamed data().
along can be used as LHS
Remove dummy value from buffer.
Move assign_swap to core/utility
Rationalize swap on different terminal

Swapping terminals of different layout can't be done without
some copies. Instead of forcing buffer to have a non standard
swap interface, we make nt2::swap handles those cases.