The 'motifMatrix' function transforms the metadata columns of a
'VRanges' object, as returned by the 'mutationContext' function, to a
matrix of the form 'motifs x groups'. This constitutes the bases for
the estimation of the signatures. By default (with 'normalize' set to
TRUE), the counts are transformed to frequencies, such that the sum of
frequencies of each group equal 1. Otherwise (with 'normalize' set to
FALSE), the counts for each mofis in a group is returned.