Make Opfi available on Bioconda

[alexismhill3]

Since Opfi has several software dependencies, all of which can be installed with the conda package manager and Bioconda, it makes sense to have a Bioconda recipe for Opfi as well. This simplifies the installation procedure, since a single conda command can now install Opfi and all of its dependencies into a clean environment.

This PR makes several changes related to the new conda installation method:

• Adds instructions for installing with conda to the README and RTD. Also makes install instructions consistent across the two documents.
• Deprecates use of the modified/custom version of PILER-CR with Opfi. Users are now instructed to install the official version (if installing with conda, this is done automatically). Additional details describing the rationale for this change will be in a forthcoming PR to remove the PILER-CR source code from this repository.
• Cleans up dependency documentation. Adds a more detailed dependency guide to RTD, including a simple comparison chart for supported homology search programs. Removes confusing optional vs required dependency distinction (the user is now instructed to install all listed dependencies, and they are required for all unit tests to pass).
• Tests are now agnostic to PILER-CR version (official or modified), whereas before some tests required the modified PILER-CR version be installed.

This resolves #199, resolves #200, resolves #201, and resolves #202