refactor ExperimentBuilder and validator for improved validator isolation

[bkmartinjr]

Fixes #215
Fixes #179

Changes:

• Break the ExperimentBuilder into two parts:
  • ExperimentSpecification - read-only declaration of Experiment configuration. May not contain information derived from the AnnData files.
  • ExperimentBuilder - the accumulated state used during the build process, derived from AnnData input files.
• Refactor the validator to only use the ExperimentSpecifications, increasing isolation from the build process. As a side effect, this fixes validator skips some assertions in standalone mode #179
• Minor fixes to test_builder which was incorrectly creating AnnData's (this cleans up all of the warnings generated during the test execution)
• Added attrs dependency (preferred over dataclasses, and used to create ExperimentSpecification)

[bkmartinjr]

Note: tests are failing due to the mismatch between the current state of tests in main and the current census build, specifically around the shape of presence. Will be resolved when we do a new build.

[atolopko-czi]

Very nice cleanup of numerous hacks!