refactor rbhtfinder

[ggutierrez-sunbright]

Thanks to @simonbray, we have a python3 version of rbhtfinder
we need to refactor and modify the script so it adheres to the coding standards and general design of rdock-utils, add tests for it, and make it available for execution after installing the rdock-utils pacakge.

the fixtures provided by @simonbray are for this invocation arguments:
rbhtfinder -i rbhtfinder_input.txt -o rbhtfinder_output.txt -t rbhtfinder_threshold.txt -f column=4,steps=3,min=-10.0,max=0.0,interval=5.0 -f column=6,steps=5,min=1.0,max=5.0,interval=5.0 --max-time 1 --min-perc 1.0 -v 5 --header