Wiki article about element indices

[holke]

Feature request

Write a Wiki article about element indices similar to the article about tree indices https://github.com/DLR-AMR/t8code/wiki/Tree-indexing

Is your feature request related to a problem? Please describe.

We often get question regarding the element indices and tbh for an uninformed user it can be confusing when to use which index.

Describe the solution or feature you'd like

Provide a well structured Wiki article with all the necessary information about element and ghost element indices and how to use them.

Describe alternatives you've considered

N/A

Estimated priority

"Priority: medium" Should be solved within half a year

Additional context

This could be a good start:

"

1. Global index = all elements on all processes enumerated from 0,...,N-1
2. local index = all elements on the current process from 0,...,n-1 global = local + number_of_elements_in_all_the_processes_of_lower_rank = t8_forest_get_first_local_element_id (forest)
3. tree local index = all process local elements of the current tree from 0,...,t-1 local index = tree local index + t8_forest_get_tree_element_offset
4. linear id = hypotetical index of the element in a uniform grid. Mostly used internally

Most of our functions require a tree id and a tree local index as input.
When iterating over the elements in a mesh we recommend to iterate over all local trees and for each tree to iterate over its elements.
"