100% test coverage

[paulmelnikow]

For a library like this, 100% test coverage ought to be a given.

Here are a few reasons:

1. Tests beget tests – having tests in place ensures that tests get written when code is changed or new paths are added.
2. Tests that cover the edge-case code ensures the code for those edge cases actually works.
3. Once we reach 100% we can also set pytest to enforce it. That way we'll know that all new code is covered without having to dig through coverage reports.

I've made some progress, and coverage is up to 79%!

In the meantime, until the projects reaches 100%, all new code should have 100% coverage.

[paulmelnikow]

After merging #81, Coverage is up to 87%!

I split out the remaining gaps into separate issues. They all involve a little bit more than testing:

• find_rigid_transform + find_rigid_rotation: Test and refactor #77
• rodrigues: Test and refactor #78
• rotate_to_xz_plane: Test and refactor #79
• Remove _temporary.setter_property #80