Improve split-sequences
Improve split-sequences

For the new sequence search we will want to split the sequences into
chunks depending up on the resulting file size. This makes the searching
much faster and decreases the required memory, if we choose the correct
file size. This reworks the split-sequences script to allow for this
option. It isn't a perfect estimation as it doesn't include the newlines
that will be included, but is good enough.

I felt that having 3 conditions was enough that I needed to create
proper objects for the data, which does increase the amount of code, but
hopefully makes the logic easier to follow at least.

RNAcentral/rnacentral-import-pipeline,RNAcentral/rnacentral-import-pipeline,RNAcentral/rnacentral-import-pipeline,RNAcentral/rnacentral-import-pipeline