prepare_bound_bound_rate_matrix should work with one shell at a time

[sonachitchyan]

Describe the bug In nlte_rate_equation_solver prepare_bound_bound_rate_matrix is designed for creating the bound-bound rate matrix for multiple shells at once, which is might not necessarily be the fastest way.

To Reproduce

Screenshots

System

• OS:

  • [ ] GNU/Linux
  • [ ] macOS

• Environment (conda list):

Additional context