Parallel version of coordinates to maps

[RaphaelMeudec]

Closes #17

[jeromedockes]

the failure is due to the memmap keeping the file open; and there doesn't seem to be a great way to close it -- we can use memmap._mmap.close but it relies on the private _mmap attribute, or on the memmap's finalizer by adding del memmap but that relies on the fact that cpython uses reference counting for gc, and it assumes no exception is raised. all in all if we plan to store all maps in memory anyway the option you posted here is probably a better tradeoff between speed and complexity? sorry for the back and forth

[jeromedockes]

otherwise what is done in #19 should work

[jeromedockes]

@RaphaelMeudec I opened a PR on your branch that fixes the windows issue and also allows the user to provide a file path for the output, making it possible to use the function without loading all the brain maps in memory:
RaphaelMeudec#1

if it looks OK to you, you can merge it and then I will merge your PR into the main branch. Or if you don't have time to look at this now, I can merge it directly into main (your commit will still appear in the git log of course)