Make the output of point2grid to CF complaint NetCDF

[hsoh-u]

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Describe the Problem

point2grid does not add the global attribute "Conventions" into the output file and saves the observation time to variable attributes. There was a case the global attribute "Conventions" was added without "time" variable. The time variable should be added to be CF complaint NetCDF which MET can handle. Otherwise, the observation time will be 1970-01-01 (default from the unix time 0). One alternative is specifying file_type to NETCDF_MET to get the right time information.
With multiple -field arguments, the handling time information at CF-complaint NetCDF need to be improved: read from "time" variable and allow to override time from the variable attribute which is not CF complaint.

Expected Behavior

Provide a clear and concise description of what you expected to happen here.*

Environment

Describe your runtime environment:

1. Machine: Linux Workstation
2. OS: RedHat Linux
3. MET 9.0

To Reproduce

Describe the steps to reproduce the behavior:

1. Run point2grid
2. Run plot_data_plane and see the mesaage

It should not show the warning
WARNING:
WARNING: NcCfFile::open() -> could not determine the valid time, using 0.
WARNING:

Relevant Deadlines

NONE.

Funding Source

NONE.

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