Simplify read mapping file extension code in rules.

[LeeBergstrand]

Problem Description:

There is a more straightforward to account for multiple extensions rather than the code below.

read_mapping_files= temp(expand("{{sample}}/{{step}}/polypolish/input/{{sample}}_input.fasta.{ext}", ext=READ_MAPPING_FILE_EXTENSIONS))

Problem Solution:

Use this instead:

read_mapping_files= temp(multiext("{sample}/{step}/polypolish/input/{{sample}}_input.fasta", *READ_MAPPING_FILE_EXTENSIONS))

[jmtsuji]

Closed via #125