A named list of numeric matrices. Each matrix has columns
A, C, G, T and one row per motif position. List names are motif
identifiers. Each matrix carries the following attributes:
- name
Motif name from the header line
- w
Motif width (integer)
- nsites
Total counts per position (numeric; NA for simple-format files)
- format
Sub-format detected: "jaspar" or "simple"