ENH: Reduce BSplineFieldmap memory usage

[effigies]

Changes proposed in this pull request

Was looking at the maths.pyx thing to see about splitting it out into its own package. Noticed some get_data() uses in BSplineFieldmap. get_data() is supposed to be being deprecated any day now... and the two calls to it are sub-optimal.

The first can just be replaced by the image shape, which is just read from the header and doesn't require loading the data.

The second can use get_fdata, and if we use caching='unchanged', then it won't store a copy of the array internally, so it can get garbage-collected after filling _data.

I don't expect this is going to have a significant effect, but just while I was there.

Documentation that should be reviewed

[oesteban]

When I revise the implementation of fieldmaps I'll rid fmriprep of Cython and this buggy implementation. I know there is a pure python implementation on scipy that fits the needs. Meanwhile, this is a great idea.