Use `get_bval` and `get_bvec` to get bval/bvec files

[arokem]

dmriprep/dmriprep/io.py

Line 48 in c16844c

bval_file = [d.path for d in dwi_files if d.filename.endswith('.bval')]

The layout object should have these, although you might have to look in several places (because of the inheritance principle)