Issues with new GLS method #124
Description
There are problems regarding implementation of the generalized least squares (GLS) method for channel combination.
In op_getcoilcombos, averages must not be combined in order to use the GLS method. There should be a check added in the beginning to verify the input data has an averages dimension to avoid getting an error.
In op_addrcvrs, using the GLS method requires four inputs to the function. This is inconsistent with the other methods, all of which calculate the coil phases and amplitudes within the function if less than four inputs are provided. If you attempt to use the GLS method with less than four inputs, however, the output twix.fids and twix.specs will be arrays of zeros.