NP summations - how deal with non detects?

[ehinman]

Is your feature request related to a problem? Please describe. There is a scenario where all of the nutrient subspecies used to add up to a total N or P could be below detection (so adding detection limits or variants on detection limits) and add up to a total N or P that is above a criterion. This is probably quite rare, but would be inappropriate. Additionally, is it wise to estimate a total based on uncertain detection limits? Questions to consider.

Describe the solution you'd like The NP summation function should indicate/flag if this ever happens.