[FC64] filter implementation

[Martin Wood]

To Ralph mainly...but if anyone else is interested

i've commited some changes to the SID branch as i've been trying to figure out 
the best way to structure the filters and actually implement it.

For now each filter type has its own class : Low Pass, Band Pass and High Pass.

I've written a basic implementation for each based on the RBJ filters :

http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt

So..the voice and filter routing is now switched on...but its all quite buggy 
atm. The low pass kind of does something reasonable as long as you dont fiddle 
with the Q. The whole mixing thing needs working out properly as well so that we 
dont get clipping when running the filters with high Q levels..but we dont want 
to make the volume too quiet..

What would be good is some kind of test code in prg format that we can load into 
an existing emulator that has got a good sound implementation so we can compare 
the output.

Hopefully i'll have some time this week to do some more, but i thought id get 
this code in so we have something to work on.


Martin