ketos segtrain no longer has `--append` option/model slicing functionality

[christopherDT]

Hello @mittagessen,

The training docs mention being able to slice off the final layers of a previously-trained model to speed up inference when alphabets are mismatched between the previous model and the training set. However, the segtrain() function in ketos.py does not include this option, and the command line throws Error: no such option: --append when attempting to include it. Per our direct correspondence, it seems this functionality may have been lost when porting the code from a previous version.