Add support for multi-coil acquisitions

[cncastillo]

Support for multiple coils, with their corresponding sensitivities, to enable parallel imaging. As a second step, we can start thinking in pTx.

[cncastillo]

After scanning a phantom with the Siemens MAGNETOM Free.Max (software XA50), and trying it with Koma, I have seen some problems:

• plot_signal plots the raw data incorrectly, the coil noise measurements there, and the coils are concatenated.
• When I read the .mrd file below, I can not reconstruct the image, even if I read the corresponding Pulseq file for the k-space coordinates.

PulseqEPITest.zip

The second bullet point could be caused by not using extensions, and the dimension of the data is not interpreted correctly in the .dat (Siemens raw data format). To get from the .dat to .mrd I used: https://github.com/ismrmrd/siemens_to_ismrmrd.