Make Palmer Indices ready to use for research

[lukruh]

Hello, it's written in the disclaimer, that the Palmer indices are not ready to use for research. Also I found a few open issues regarding to the implementation of the palmer indices. I've read the palmer and wells papers and can see that it's not just a trivial equation to implement, but I wonder what the specific tasks would look like to make this ready to be used for research. I'm not an expert on palmers indices, but I've worked with drought indices before and would prefer to work on the index implementation in this library rather than switching to another existing library (i.e. scPDSI in R). If there is any chance to get this done in the near future.