Allow for ExactPredicates to be toggled

[DanielVandH]

Fixes #122

• Adds Preferences.jl as a new dependency
• Fixed the unused orient(p, q, r, s)
• Added getz/_getz/getxyz/_getxyz
• Added a missing license from ExactPredicates.jl
• Allow users to toggle off ExactPredicates.jl using Preferences.jl, allowing simple determinant calculations to be used. This won't make much difference in most cases I imagine, except for the fact that when a user needs ExactPredicates.jl they will only be able to find out through an error (if they toggled it off).

TODO:

• [x] Document how to toggle it off
• [x] Add a warning to users about consequences of not using ExactPredicates
• [x] Put validate_triangulation into the package to allow users to make sure their triangulatiosn are sensible if they want that safety.
• [x] Fix CI to test preferences

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.67257% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.57%. Comparing base (35b7ec2) to head (cea95ca). Report is 2 commits behind head on main.

Files	Patch %	Lines
src/validation.jl	98.10%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #131 +/- ## ========================================== - Coverage 95.32% 94.57% -0.76% ========================================== Files 92 93 +1 Lines 9139 9785 +646 ========================================== + Hits 8712 9254 +542 - Misses 427 531 +104 ```

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