Add additional atlas validation functions.

[alessandrofelder]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

What does this PR do?
Improves our validation functions based on experiences with

• atlas names needing to be lowercase make kim dev atlas names lowercase #446
• wrapup rescaling float64 data too naively Add all Kim's DevCCFv1 atlases #441 (comment)
• some atlases having differing annotations for the same region in the opposite hemisphere (e.g. [BUG] Nadkarni mouse lemur regions need symmetrising #451 and Hemisphere improvements to axolotl atlas #438 )

so the above issues will be flagged automatically during packaging in the future, when we merge #445

Also, refactors our validation tests for simplicity, and makes sure we are actually testing validation functions on a valid atlas (for the tests we expect them to pass).

[alessandrofelder]

Note that one of the new validation functions catches asymmetry in labels on #451, a real-life example.

Failed validations:
('validate_mesh_matches_image_extents', "('Mesh coordinate 20156.5 and annotation coordinate 22932.0', 'differ by more than 10 times pixel size 91.0')")
('validate_annotation_symmetry', 'Annotation labels are asymmetric.')

Mesh extent validation is also expected for that atlas (there are some spurious pixels in the original data).