Update test-mass noise model

Closes #136 (closed)

• Add option testmass_shape (similar to laser_shape) to add low-frequency relaxation to test-mass noise model on Instrument. Default remains at 'original' , one can enable this extra low-frequency component with 'lowfreq-relax' .
• Add missing high-frequency component to test-mass noise (both original and lowfreq-relax models).