Adds unit tests for terrain importer to check sphere collisions (#69)

# Description

Adds in unittests for terrain importers. check that all three import
function work and generate meshes of correct size and the balls dropped
on mesh do not fall through it.

Fixes #54

## Type of Change

- New feature (non-breaking change which adds functionality)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file

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Signed-off-by: James Smith <142246516+jsmith-bdai@users.noreply.github.com>
Co-authored-by: James Smith <jsmith@theaiinstitute.com>