LegacyConvertedEnhancedMRImage error with color dicoms

[aporter-turing]

I am attempting to use the converter to change a series of dicoms into an enhanced dicom. The dicoms are in RGB format but aren't being recognized during conversion.

    if array.ndim > 2:
        if planar_configuration is None:

           raise ValueError(
                Planar configuration needs to be specified for encoding of 
                color image frames.)
           ValueError: Planar configuration needs to be specified for encoding of color image frames.

I noticed that further along the converter pipeline the parameter planar_configuration is not given during this segment but the dicoms have the header ds.PlanarConfiguration I think this PR fixes the error above.

[CPBridge]

As discussed on #318, legacy conversions can only use monochrome frames. #321 adds a check to enforce this to give a better error message.

Closing as complete