tomtom fails without altid

[snystrom]

Issues
Main reason for this feature is to avoid rare scenario where two motifs may have identical names (like if the user joined two data.frames).

Possible Solutions

• It may be that tomtom doesn't allow identical named motifs as input (haven't checked this). If this is the case, then I can drop requirement of altname and instead check & error if names are non-unique?
• add altname columns to everything initialized with NA_character_ if altname column does not exist.

[snystrom]

Test whether importTomTomXML has same issue