Implement Nexus Writer for t-route formatted QLAT outputs

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This PR implements the ngen::utils::nexus_writer base class with ngen::utils::nexus_csv_writer and ngen::utils::nexus_netcdf_writer derived classes. This implements part 1 of #605.

See NOAA-OWP/t-route#636 for the output schema. Output file names are formatted as: {%Y%m%d%H%M}NEXOUT.{nc|csv}, based on t-route's parsing logic.

Usage

The following modifications should be made before using this:

• Before running ngen, export the NGEN_QLAT_FORMAT environment variable to be either netcdf or csv.
• in your routing config file, ensure forcing_parameters.qlat_file_pattern_filter is set to *NEXOUT.csv or *NEXOUT.nc, depending on the format you output.
• Optional, but recommended: set output_root in your realization config so that the outputs are aggregated under a single directory.
• Ensure your forcing_parameters.qlat_input_folder points to the same location as your realization config's output_root, if set.

Additions

• Adds ngen::utils::nexus_writer
• Adds ngen::utils::nexus_csv_writer
• Adds ngen::utils::nexus_netcdf_writer
• Adds environment variable checking for user to choose QLAT output. For example, setting NGEN_QLAT_FORMAT=netcdf will output NetCDF files, or otherwise CSV files.

  This is a temporary workaround until a decision is made on whether this should be set in the realization configuration files.

Changes

• Modifies CSV outputs to conform to NetCDF format.
• Modifies src/NGen.cpp to include the logic for using ngen::utils::nexus_writer.
• Modifies src/NGen.cpp to prevent destruction of nexus_collection, since the flowpath/segment IDs are needed for method calls to nexus_writer.

Notes

• If NetCDF is not available at compile-time, but NGEN_QLAT_FORMAT is set to "NetCDF" at run-time, then a std::runtime_error will be thrown when any of nexus_netcdf_writer's methods are called.

Testing

• Adds unit tests for ensuring output formats are correct by parsing temporary outputs for both CSV and NetCDF.

Todos

• Unit Testing for Nexus Writers
• Corresponding t-route PR (waiting on inland hydraulics)
• MPI handling (?)

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

Testing checklist (automated report can be put here)

Target Environment support

• Linux

[mattw-nws]

Ah.. can we just delete this block now?

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Probably replaced by #744

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@hellkite500 @donaldwj does it seem reasonable to close this?