Add new mode to FakeSeriesSource
This MR adds two new modes of operation to FakeSeriesSource:
- 'const_int' or 'constant_int': provides a source pad with a constant stream of integers, as specified by the user
- 'const_float or 'constant_float': provides a source pad with a constant stream of floats, as specified by the user
Unit tests for each of these modes have been added as well.
These two modes are useful for testing various calibration-related elements in order to mimic things like front-end oscillator channels or front-end state vector channels.