[Stk] small improvement to Filter::phaseDelay()

[Andreas Sumerauer]

Hello,

I have been recently looking at the filter class and found a small 
improvement to the  Filter::phaseDelay() function. I hope you like it.

This:

inline StkFloat Filter :: phaseDelay( StkFloat frequency )
{
(...)
   StkFloat real = 0.0, imag = 0.0;
   for ( unsigned int i=0; i<b_.size(); i++ ) {
(...)
   real = 0.0, imag = 0.0;
   for ( unsigned int i=0; i<a_.size(); i++ ) {
(...)
}

can be replaced by:

inline StkFloat Filter :: phaseDelay( StkFloat frequency )
{
(...)
   StkFloat real = b[0], imag = 0.0;
   for ( unsigned int i=1; i<b_.size(); i++ ) {
(...)
   real = 0.0, imag = a[0];
   for ( unsigned int i=1; i<a_.size(); i++ ) {
(...)
}

It saves a bit of CPU bandwidth by utilizing the trigonometric 
identities cos(0) = 1 and sin(0) = 0.

best regards

Andreas Sumerauer