Also change SIFlaot to pass through 'None'

This solves the issue that not in all cases the
value of 0 can understood as 'not set'.

adding in fsig command (not parsing yet). See example test_fsig.py in bin folder. Also made component variable an optional argument for xaxis and x2axis which will break previous scripts. Did this as when setting the parameter to tune, the Param object contains whatever component owns that parameter so no need to pass it twice. Also stops someone passing a parameter not for the component stated.

adding in new photodiode object which has f1,phi1,f2,phi2,etc., properties so that they can be tuned/put/set. Not fully transffered over from previous one yet.

Large amount of changes to the param object which now include being able to put and set parameters and commands. Also adding the modulator component.