Refactor DataProvider::get_values interface and implementations to avoid excess allocation and copying

[PhilMiller]

https://github.com/NOAA-OWP/ngen/blob/dd664668975e84a21d58c79300248210a0a256bf/include/forcing/DataProvider.hpp#L87

The member function signature in the DataProvider interface demands the allocation of a temporary vector and the likely copying of data from that temporary vector by the consumer.

The signature ought to be

virtual void get_values(boost::span<data_type> output, selection_type const&  selector, ReSampleMethod = SUM);

We could phase the refactoring by

• adding this as an overload to the interface, with a default implementation that just calls the existing get_values and copies the data
• Implement for various DataProvider implementations
• Exploit in DataProvider::get_values callers