Cleanup math and LastUpdate fix

[gztensor]

Description

• Remove recursion, indexing, assertions, and zipping.
• Remove math.rs functions and corresponding tests that are not used
• Add a test for mismatching LastUpdate and Weights => Crashed TestNet recently
• Fix TestNet crash root cause:
  • Fix get_last_update (will always return proper length vector)
  • Fix LastUpdate vector update in add_neuron

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[shamil-gadelshin]

I noticed that you have two policies for replacing assert_eq!(len1, len2) - safe implementation and skipping. Is it intentional?

Another question - do we have all this math covered by tests?