Permutation computation refactoring

[chrisrichardson]

Generalise face permutation computation to get ready for mixed topology meshes.

• reduce duplicated code in face permutation computation for tet, hex
• adds computation for prism, (should work for pyramid too)

[chrisrichardson]

Would be good to add a test - if anyone can think of a good one @mscroggs @jpdean ?

[mscroggs]

To answer the why: min_v wants to be the number of vertices anticlockwise away the minimum numbered vertex is. If the vertices were numbered:

3 -- 2
|    |
0 -- 1

We wouldn't need to do the swap, but as we use the numbering:

2 -- 3
|    |
0 -- 1

we therefore need the swap

[chrisrichardson]

Thanks @mscroggs . Maybe you can add something in-line to document this.

[chrisrichardson]

Fixes issue #3208