New functions for next version

[srcorsi-USGS]

Two suggestions for new functions that will help vet chem/endpoints for potential exclusions. They will provide similar, but slightly different views of the same thing.

1. Scatter of ACC values for each chemical: beta version is in the passive repo ./R/analyze/graph_ACC_by_EP_group.R as a script. Need to functionalize and generalize so it can be used for benchmarks also.

2. Cumulative distribution curves for endpoint/benchmark ACC values. Example script for the passive ms is in that same directory as graph_ACC_distributions.R.