Create new branch with sequence collections backend prototypes

[GavinHuttley]

call the branch seq-collections-refactor

• create src/cogent3/core/new_alignment.py put the classes and auxiliary functions for sequence data collections / views in there
• create tests/test_core/test_new_alignment.py and put all the tests in there
• change all your imports so the tests pass

[fredjaya]

@GavinHuttley Branch is on my fork here https://github.com/fredjaya/cogent3/tree/seq-collections-refactor. Don't have permissions to add to the main repo though.

Noting that a lot of the WIP AlignedData methods were removed in the latest commits

[GavinHuttley]

thanks @fredjaya!

Can you make a PR from your repo to the new seq-collections-refactor branch I created.