Support returning seqcoding using other aggregation functions.

[akanakia]

Currently def seqcoding(self, seqs, align=False) only supports generation of res_codings using res_to_seq(mode="mean") in line:

AbLang/ablang/pretrained.py

Line 112 in 6fc320d

seq_codings = np.apply_along_axis(res_to_seq, 2, np.c_[np.swapaxes(residue_states,1,2), lens])

Consider updating the the seqcoding method to also allow changing the aggregation mode of "sum", which is supported by res_to_seq.