Add shape check to MetaResult initialization

[tsalo]

Related to a bug in #304 that only shows up when you try to get a map from the PermutedOLS estimator's results. That bug should raise an error during creation of the MetaResults object instead.

[tsalo]

I think the main obstacle here is going to be supporting different types of maskers. Currently, Datasets can't work with atlas or sphere masks, because these don't have mask_img attributes, but presumably we'll want to support those in the future... in which case, checking the data shape is a little harder here.

I might just write a fix that only works with NiftiMaskers for now.

[tsalo]

MetaResults should support multiple kinds of maskers right now, so the above idea might not work.

[tsalo]

This will be much easier to do if https://github.com/nilearn/nilearn/issues/3296 is implemented in nilearn.