Angle Modulation Refactor

- Moved AngleModulation to cilib.problem package
- Extracted the bit generating function from the AM problem class
- Any ContinuousFunction can be used as a modulating function
- Added a modulating function with variable amplitude
- Implemented the default angle modulation sampling method
- Implemented a MinMax sampler
- Added a MinMaxAngleModulation decorator
- Removed functionality for multiple bit generating functions
- Updated xml examples
- Updated test cases
- Fixed a small bug in the StringBasedDomainRegistry copy constructor

Signed-off-by: Bennie Leonard <bennie.leonard@gmail.com>