series: change the sfinae style of the generic ctor and some doc fixes.
poisson_series: a minor style change in the sin/cos implementation.
build system: add another useful debug compiler flag for GCC (clang as well possibly).
Roll back current uses of inheriting ctors.

A couple of reasons for this:
* it seems like the support across the three compilers is uneven (only
  GCC 4.9 seems to get it right in all cases),
* I am not sure I understand completely the semantics of this :)

Probably better to revisit at a later date.