Return warning when decay function is above 0 at maximum cutoff

[trevorgerhardt]

R5 now allows decay functions that do not drop all the way to 0 (see https://github.com/conveyal/r5/issues/677). R5 should return a warning to display if the decay function is not close to 0 at the maximum cutoff.

Originally reported at https://github.com/conveyal/ui/issues/1494