model.apply fails for a model sequence when seq_in is not equal to 'data'

[grlloyd]

For model sequences seq_in must equal 'data' or model.apply fails. Example error when seq_in = 'names' for filter_by_name as the first step in a sequence:

Error in (function (classes, fdef, mtable)  :
  unable to find an inherited method for function ‘model_apply’ for signature ‘"model_seq", "filter_by_name"’

[grlloyd]

Documentation for struct needs to be updated and a check added to model_seq that throws an error or warning if seq_in doesn't equal 'data'