added whitening to the _BaseCorrFunc, and moved noise generation out to a shared utility function.  NOTE spotted bug that was causing noise fieds from _CorrelatedNoise instances to be two-fold rotationally symmetric... Fixed, at cost of more random number calls
(#352)
smartened up getting of cached rootps with utility function, avoiding duplicationg
(#352)
added COSMOS test, will add whitening test too...
(#352)
added whitening tests
(#352)