Updates to gwmat.injection Module and Addition of gwmat.nr_injection Module

1. Major updates to the structure and organization of gwmat.injection:

   • Refactored the hp/hc generation function into smaller, modular components for improved flexibility and code clarity.
   • Default parameters are now initialized separately and can be accessed as an attribute of the class instance.
   • Renamed several functions for better consistency.
   • Changed the keys f_low and f_high to snr_f_min and snr_f_max to avoid confusion with f_start/f_lower and f_final during waveform generation.

2. Documentation and tutorial notebooks:

   • Updated to reflect the recent changes.

3. Adherence to PEP8:

   • All modules have been formatted using black and checked with pylint for possible issues and PEP8 violations. Consequently, all modules now adhere to PEP8 guidelines as much as possible.

4. Additional updates:

   • Introduced a new module, gwmat.nr_injection, along with a corresponding tutorial notebook for injecting Numerical Relativity (NR) waveforms from SXS simulations.