Added the correct extra HP output to original RK4 algorithm, and provided an oversampled 4 times version + a 0df version

This new algorithm for a transistor ladder filter is stable for all the cutoff frequencies, uses the TPT structure which ensures by design that any kind of cutoff frequency modulation will not create any artefact. And it uses a trick which makes it 0df on the linear side only, so that it is less CPU consuming than standard 0df filters using implicit solvers, but with its frequency response being the one expected.

I have replaced the previous Fundamental VCF HP output formula with the correct one, and I have added a commented instruction which could be used to provide a BP output as well if wanted.

Note : I have not tested yet the submitted change, but these formulas are the one I use all the time in my own Moog-like / transistor ladder filter algorithms, such as the one in Sonic Academy ANA 2.

For more information about the formula I have used, see there : http://www.kvraudio.com/forum/viewtopic.php?t=207647