Convert string None to Python None

[tyamaguchi-ucla]

Description

As described in #208, it appears that the conversion of the string "None" to the python None object wasn't implemented in the parse_combine_counts_args function unlike the ‎parse_combine_counts_fw_args‎ function. This results in the error shown in #208. Therefore, it seems that the phase_snps step is not treated as optional when fixed_width = False.

Closes #208

Testing Results

When

[combine_counts]
# Path to phased file; leave as "None" to run hatchet without phasing
phase = "None"

It successfully passed check without failing.

NO PHASING FILE FOUND at /path/phased.vcf.gz. Not including phasing in binning process.
[2024-May-01 21:28:51]# Parsing and checking input arguments

@mmyers1 @balabanmetin Please let me know if you want me to run additional tests.

[mmyers1]

simple change, looks fine