Improving finite IR class (SetDirac) and re-implementing a little improved StringToDoubleVec parser, which is still far from solid

Moving from old atomic implementation to c++11 atomics and providing some copy constructors for msvc14 compat

# Conflicts:
#	src/ITAISO9613.cpp

Added function for the calculation of the attenuation absorption in decibels according to ISO 9613-1.
The air attenuation can also be calculated as an energetic factor between 0 and 1.

Cleanup, refactoring and changing buffered audio file writer behaviour to enable lazy file path setting

Refactoring sample buffer fading definitions, now all in ITABaseDefinition and with appropriate namespacing and enum declaration

Refactoring fading definitions, now all in ITABaseDefinition and with appropriate namespacing and enum declaration