Streamline installation

[desilinguist]

Right now, installation includes installing the conda package for rstfinder and then installing the PyPI package for python-zpar. This is because python-zpar is only available via PyPI. To make it easier for uses, we should either.

1. Make PyPI the primary method for distributing rstfinder instead of conda.
2. Remove the dependency on python-zpar (see change parser #53 ) and use another parser that's available via conda.

[YTZ01]

Hello, I have a problem in running this line'''convert_rst_discourse_tb ~/corpora/rst_discourse_treebank ~/corpora/treebank_3'''. I'm wondering the PDTB dataset in your setting is PDTB-v1（2019） or PDTB-v2（2020）, cause I downloaded the dataset from LDC, but it doesn't have a 'parsed' file under it, only data，docs and tools, index.html. Have you met this issue? @desilinguist