Constraints patch

[aniketp]

@StuartLittlefair , @bmorris3 . This is in relation to the results I got from profiling the code. I have made some changes regarding that. Adjusted the code slightly so that memory usage is minimized, the properties _all_constraints and constraints were being used repetitively . Also separately added the part which can append AltitudeConstraints in b.constraints if initially the constraints was not empty and there was no instance of AltitudeConstraint (default 0 deg) in it.