modify `decodeBedXY` to handle tracks that contain `p-values`

[fossilfriend]

will need to at least do the following:

• [ ] determine all possible fields that could contain p-values from the schema file (see slack chat history)
• [ ] if file contains a p-value add pvalue attribute to the BedXYFeature
• [ ] if file contains a p-value, calculate -log10p and add to the BedXYFeature as neg_log10_pvalue attribute