configure: add ability to set c++ standard version

Default to C++11 for now. Also check if the compiler supports
constexpr and if so we use that where it is needed.
Prefix uses of pow() with std::

This avoids the conflict with overloads defined by CONTAINERFUNC
in numerics
NIX IO: automake support (discovery, cppflags)
[nix] create file and save initalial metadata
[nix] Move creation, closing into NIO struct
[nix] Move metadata writing to NixFile
SpikingNeuron: replace mu for micro- with u
[nix] write relacs::MetaData on file close
[nix] write data traces as DataArrays
[nix] initial work on event saving
[nix] Complete event saving
[nix] Link events, traces to stimuli times
[nix] Only link valid events to traces
[nix] style changes
[nix] Link top-level metadata to top-level block
update of dataExtent method calls to match new nox version
add NIX_LIB and NIX:LDFLAGS to fix linker problem
change nix::createProperty call to latest version
Merge branch 'nix_io'
Merge branch 'master' of https://github.com/relacs/relacs