Updating code base to work with new yaml-cpp API.

So far only the yaml unit tests are functioning.
Updated accumulator test case with new interface.
Updated CIC unit test code.
Added external stimulus file for dynamic configuration in cic unit test.
Updated CORDIC module code.
Removed down_converter unit test. Cordic unit test replaces this.
Updated basic_filter unit test.
Updated PhaseAccumulator module.

Module now has independent input and output widths.
First working version of usrp_v1 model with new parser.

1. Uses cic generator that reads in sdr.yml to generate a cic filter at compile-time.
2. Can turn on/off bit-pruning algorithm.
3. Added ADC sampling model that introduces noise based on the provided SNR.
4. Added stimulus.yml file for any parameter that's adjustable at runtime.

TODO: Update unit tests to match new file structure.
Updated accumulator unit tests to match new structure
Updated cic unit tests to match new structure
Updated cic unit tests to match new structure.
Removed old filter_stage unit test.

Filter stages are no longer implemented at the higher-level.
Updated readme to reflect recent modifications.