Express frame rate as ratio of two integer values throughout code
using struct Fraction

Add support for defining fractional frame rate from command line

Shorten parameter names to NNPFC to save space in user manual (an maybe make them easier to type).

No specific table formatting is required for NNPFC anymore.

- Add the control for that flag in the cfg file & update the manual doc
- Implement checking at encoder side to ensure that when sending an update the base filter is already present
- Implement detection at decoder whether the given base filter is a new or a repetition filter.
- Implement checking at decoder side to ensure that when receiving an update filter, the base filter has been received earlier / already present.

JVET-AC0344: Signal 2 new syntax elements, nnpfc_extended_patch_width_cd_delta_minus1 and nnpfc_extended_patch_height_cd_delta_minus1 when nnpfc_constant_patch_size_flag is false, and only signal nnpfc_patch_width_minus1 and nnpfc_patch_height_minus1 when nnpfc_constant_patch_size_flag is true