HybPiper version 2.1.8

• Add new subcommand hybpiper filter_by_length, used to filter the sequence output of hybpiper retrieve sequences by absolute length and/or length relative to mean length in target file representatives. This is done on a per-sample/per-gene basis, rather than the sample-level filtering available in hybpiper retrieve_sequences. See wiki for more information.
• Update the regex used to check target file fasta header formatting, to capture scenarios where a name contains multiple dashes and also ends with a dash.
• In the fix_targetfile.py module, remove the import of Bio.Align.Applications.MafftCommandline and call MAFFT via subprocess (see issue#147).
• In the gene_recovery_heatmap.py module, cast the dataframe from the seq_lengths_file to object dtype to avoid a deprecation warning .
• Add option --no_heatmap to command hybpiper paralog_retriever (see issue#150).
• Fix an Exonerate-related debug message in exonerate_hits.py.