Ensure forcing path when file_pattern configured.

[robertbartel]

TL;DR

This change enforces requiring a path whenever file_pattern is present in forcing params in a realization config.

Explanation

The addition of NullForcingProvider included support for the absence of a path value from forcing in a realization config.

        "forcing": {
            "provider": "NullForcingProvider"
        }

Those changes did not fully account for user errors involving omitting the path in other situations, in particular when a file_pattern is configured; e.g., doing this

        "forcing": {
           "file_pattern": "/dmod/datasets/forcing/regridded/regrid_01/csv/{{id}}.csv",
           "provider":  "CsvPerFeature"
        }

instead of this

        "forcing": {
           "file_pattern": "{{id}}.csv",
           "path": "/dmod/datasets/forcing/regridded/regrid_01/csv/",
           "provider":  "CsvPerFeature"
        }

Now an error will be thrown explaining that path needs to be set to the parent directory.