add function to calculate the twin mic pattern

Takes the amplification of the backwards cardioid (beta) to estimate the
pattern and an angle. Returns the amplification at this direction.
change standard behaviour of twinIcaToAngle

Standard is now _not_ to force angles in any direction.
fix bug in beta calculation in twinIcaToAngle

Backwards looking cardioids where not recognized and produced a wrong
beta.
Added tests for beta calculation.
fix bugs and add tests in twinIcaToAmp

Backwards looking cardioids where not calculated correctly.
There was a wrong vector mulitplication, which is now elementwise.
Added tests.