Health checks for Region Search: update duplicates_check and add a NaN cleaner.

[ColinOrionChandler]

Update duplicates_check to new @wilsonbb + @ColinOrionChandler code that keeps the last duplicate and drops the rest.

Also, need a function that finds rows with NaNs and drops them.

Or combine those into a "health check" function or the like.

[ColinOrionChandler]

Better to check this during Region Search than during ImageCollection creation. Also, it's valid for an ImageCollection to have duplicate datetimes in the case of e.g., LBT.