Check number of opportunities in destination layers used for regional analysis

[ansoncfit]

We should add assertions that destination layers used in regional analysis have fewer than 2 billion opportunities.

With more than 2^31 opportunities, access results run the risk of overflow. We recently caught this with an assertion building the grids to display regional analysis results:

java.lang.IllegalStateException: Opportunity density should never be negative.
at com.google.common.base.Preconditions.checkState(Preconditions.java:512)
at com.conveyal.r5.analyst.Grid.write(Grid.java:303)

But we should fail faster, rather than waiting to violate an assertion after the analysis is complete.