Added new simple KDE method and SEPP-derived method for stationary bg SEPP with rule of thumb fixed bandwidth.  Slightly faster and a lot simpler.
Added to validation methods allowing the SEPP model to REMAIN FIXED either after initial training or pre-defined at the start.  This is hugely advantageous for when it's a big job to train it - can save it, reload and supply to the validation code.  Most important thing is getting the time cutoffs correct though.
Added test for these methods, but it's crappy and at present takes AGES - might not even pass.  TODO: fix this.