Need ability to indicate BMI forcings and relax requirements for path/files to be input under this option

[hellkite500]

Short description explaining the high-level reason for the new issue.

Current behavior

A BMI module can act as a forcing DataProvider by advertising other model inputs as its outputs. This is currently how multi-bmi connects various BMI models together -- ngen proxies the variables between modules.

However, if one wanted to rely soley on a BMI module to provide all required inputs (forcings), ngen will still expect to be provided a forcing block in the realization configuration (for global and catchments) and that block will expect to find valid directory paths and/or files for forcing data.

Expected behavior

If a BMI module is intended to be the source of all forcing variables, then ngen should accept a configuration which indicates that a BMI module is expected to provide all forcings and it should not attempt to create any providers and should require not additional inputs.

[hellkite500]

#601 could be a solution for this issue.