[FEATURE] Multithreaded processing

[Oleg4260]

Is your feature request related to a problem? Please describe.
The program uses only 1 thread while processing data and generating the map, so it's much slower than it could be on modern CPUs

Describe the solution you'd like
I understand that this isn't easy to code but it would be very good for generating large maps since modern CPUs have 4, 8, 12 or more threads, I think it's possible to run the program in multiple threads by invoking a few instances of the same function for each thread with different coordinates given for each function.

[benjamin051000]

See #16 never mind, that's from back when the codebase was python

Excerpt from Louis in #300:

Arnis is indeed processing only a single element at a time, since elements can stretch over the entire map (e.g. streets). In combination with the element type processing order and many potentially overlapping block read and write calls at the same time, I failed on implementing multithreading on my first try. Only the saving step makes use of multithreading in the upcoming release since there are no "overlapping" write calls involved.

This is 100% on the to-do list. Right now I've been focusing my efforts on cleaning up a lot of the codebase (disclaimer: I am not a maintainer of this repo).

After that's done and the codebase is in a more maintainable state, we can look into performance optimizations.