Nilearn: Use pandas in the data loaders

[NicolasGensollen]

Since Pandas was added to the dependencies, we can use it in the data loaders to read CSV files instead of Numpy.
See #2619

[jeromedockes]

Since Pandas was added to the dependencies, we can use it in the data loaders to read CSV files instead of Numpy.
See #2619

I wouldn't modify existing loaders; but when writing new loaders (which
should almost never happen anyway) pandas can be used when that's more
convenient

[GaelVaroquaux]

I wouldn't modify existing loaders;

I had in mind modifying loaders :)

[jeromedockes]

I had in mind modifying loaders :)

oops just saw your comment in #2619 -- ok, let's change them then :)

[achamma723]

Hello @NicolasGensollen , I'll work on this issue.

[NicolasGensollen]

Great! Thanks @achamma723 !