Allow DDLm Set categories to have keys

[jamesrhester]

As per the recently accepted multi-block principles [1], a DDLm Set category can have key data names if items in that category can be combined by merging data blocks. ddlm_validate currently flags Set categories with key data names as being in error. I suggest this is removed as an error or an option provided to ignore it.

[1] http://comcifs.github.io/accepted/multi-block-principles

[vaitkus]

Thank you for the update. The warning messages are actually generated by the cif_ddlm_dic_check script, though the ddlm_validate/cif_validate scripts do not yet properly handle Set categories with keys. I plan to implement changes to both of these scripts in the same branch.