Rethink `i_global`

[isteinbrecher]

Unify how we handle global indices.

• Unify the naming of the variables
• Try to get as much index-0 based as possible
• Try to replace i_global in the elements and nodes with mappings (check performance for that)

See also the point from #456 (comment):

• Evaluate if we want to keep the global indexing of materials / BC / functions / ... as introduced in Mesh representation as internal input file storage #633 . Alternatively, we could also think, if we want to have this also for nodes/elements Mesh representation as internal input file storage #633 (comment)