Add an example that approximates Pi.

[holke]

Closes #1801

This is in draft status since i feel it is kind of hacky and put together quickly.
However, i do not want the changes to be lost so pushed it and opened this PR.
May or may not come back to this and beautify it in the future.

@spenke91 this could potentially become a benchmark case.

We compute Pi by computing the area of the unit circle.
We refine at the circles boundary and sum up the area of the inner triangles.

Describe your changes here:

• add new example folder "other"
• add example "t8_compute_pi" that computes pi by approximating the area of the unit circle.

What i would like to change before properly merging:

• Use a real [-1,1]^2 cmesh instead of transforming the [0,1]^2 cmeshes coordinates by hand.
• Add command line options for changing the element shape.
• clean up and properly document

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