particle injection implementation + refactoring

[grzegorzbor]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 89.52234% with 68 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.18%. Comparing base (8e88445) to head (f1c1dab).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
agnpy/time_evolution/time_evolution.py	82.75%	40 Missing ⚠️
agnpy/time_evolution/_time_evolution_utils.py	90.64%	26 Missing ⚠️
agnpy/spectra/spectra.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   97.22%   96.18%   -1.05%     
==========================================
  Files          44       47       +3     
  Lines        3826     4250     +424     
==========================================
+ Hits         3720     4088     +368     
- Misses        106      162      +56     

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