Phase 1: Adds support for single operations on non-fresh ciphertexts

[christopherngutierrez]

Proposed changes

• Adds support for non-fresh ciphertext (these are ciphertexts that have been Moded down one or multiple times)
• Refactored KeyMul, Decomp, Mod, Relin, and Rescale to work with non-fresh ciphertext
• Refactored to split_last_rns_polys to use current_rns to produce last/remaining Poly objects
• Tested and passing for all single ops under BGV/CKKS on fresh/non-fresh ciphertext

Note: The PR for Phase 1 provides support for stand alone non-fresh ciphertext but does not currently support multi-ops Mod due to a secondary issue with collision of intermediate variables. Phase 2 PR will provide mutli-ops support.

Types of changes

What types of changes does your code introduce to the HE Toolkit project?
Put an x in the boxes that apply

• Bugfix (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 (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating
the PR. If you are unsure about any of them, do not hesitate to ask. We are
here to help! This is simply a reminder of what we are going to look for before
merging your code.

• I have read the CONTRIBUTING agreement
• Current formatting and unit tests / base functionality passes locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

[kylanerace]

Confirmed to be working with needed changes to support Relin operations on non-fresh ciphertext.
Pending phase 2 will support multiple modsw in order.