Add ISMRMRD import/export

[paquiteau]

ISMRMRD is the standard for sharing k-space data acquired using an MRI machine ^1. There exists Python bindings ^2 , as well as a toolbox for handling the data ^3, the examples in it are quite instructive on how we could read (and write !) ISMRMRD files.

Ideally, MRI-NUFFT should be able to read the kspace-data and trajectories in ISMRMRD files, and process them to make them compatible with our standards. Ideally, it should also be able to write ISMRMRD, for interoperability with other toolboxes.

There is also vendor conversion tools, for instance for Siemens^4