`cher_utils` uses `bgvd_model` directly

Related to #35

File cher_utils.py imports bgvd_model in https://git.jinr.ru/dnaumov/ntsim/-/blob/main/ntsim/CherenkovGenerators/cher_utils.py#L131 and uses BaikalWater directly in FrankTammFormula_quad function.

This means NTSim cannot be run without bgvd_model. We should make this an abstract dependency.