spectrum: Refactor 3GPP channel parameter generation and clarify LSP ordering

This commit refactors the 3GPP channel parameter generation logic by
splitting GenerateChannelParameters into smaller, clearer helper
functions and improving overall code structure and readability.

In addition, missing Doxygen documentation was added and clarified,
including the correct ordering of large-scale parameters (LSPs) for
different propagation conditions, which is essential for understanding
index usage in the code.

The LSP ordering follows:
- LOS:  [SF, K, DS, ASD, ASA, ZSD, ZSA]
- NLOS: [SF, DS, ASD, ASA, ZSD, ZSA]

Related refactoring and tests were updated accordingly.

Author: biljkus