Improve clair3 and Freebayes merger function

Updated classify_single_var function to account for cases where clair3 and freebayes report different variants at the same position.

Often clair3 and Freebayes do report the same variant in a slightly different notation (e.g. T to A for clair3 TT to TA for freebayes). I therefore added a check for the length of the reported variants to avoid reporting all variants twice.

The python file was tested on one of the clair3 and freebayes output files and produced the desired merged file. I did not test whether the script is still able to run in the context of the pipeline.