mincSummary imposes grouping when using method = 'mean'

[abeaucha]

mincSummary() behaves different with method = 'mean' and method = 'median':

mincSummary(filenames = ... , method = 'mean', grouping = NULL) returns an object with classes "mincMultiDim" and "matrix", and with dimensions (nfiles, 1).
mincSummary(filenames = ... , method = 'median', grouping = NULL) also returns an object with classes "mincMultiDim" and "matrix", but with dim() returning NULL and length() returning nfiles.