ImageType in mrd_image.yml

[kekeke21]

Currently, mrd_image.yml contains a field ImageType as enumeration to specify the data type of the Image field. However, it is obvious to infer whether the data is complex or not by just looking at the data. What is more important for the reader is what kind of image is stored; T1-weighted, T2* weighted, ADC, PD, B1 calibration map, coil sensitivity maps, etc. These are labels we suggest adding to the Image Header field.