NullDataProvider or ZerosDataProvider for forcing-less simulations

[mattw-nws]

Create a DataProvider that can be used in the forcing block of a realization that needs no actual data. This can be used in the case where a BMI module needs to itself retrieve forcing data, e.g. in a scenario where forcing data cannot be read by (or cannot be transformed into a format currently supported by) ngen. Specifying this provider in the forcing block would obviate the need to put in dummy/irrelevant/synthetic forcing data for such a scenario.

The simplest solution would be a NullDataProvider that has no variables and yet reports that it has valid data for all possible time values. I believe the existing APIs will allow this approach. If not, a ZerosDataProvider could be created that would publish a single variable named zero_data_provider of type double for any time value (in theory this variable could be actually wired to some formulation, though the use-case for this is questionable).

[mattw-nws]

Relevant additional info: the implemented DataProvider class needs to pretend to support a request for "data" for any time--I think the DataProvider interface allows the system to ask the DataProvider for what time range it supports--this provider needs to be willing to provide data (either no actual variables or some constant value like 0 or 1) no matter what time the data is requested.