Return iterators instead of vectors in datafacade_base functions

[ghoshkaj]

Return iterators instead of vectors in the datafacade_base function as it will be faster to return the iterators instead of making new copies of the vectors and returning those.

For example in #4876, the number of times that GetUncompressedForwardDurations and GetUncompressedReverseDurations get called increases by a large number and in order to ensure that it doesn't become a bottleneck, doing the above would be very helpful.

virtual std::vector<EdgeWeight> GetUncompressedForwardDurations(const EdgeID id) const = 0;
virtual std::vector<EdgeWeight> GetUncompressedReverseDurations(const EdgeID id) const = 0;