Feature 1630 point2grid hang

[hsoh-u]

Pull Request Testing

• Describe testing already performed for these changes:
  

point2grid empty_obs.nc G211 empty_grid.nc -field 'name="TMP"; level="Z2";' -v 10

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

MET_BIN=/Volumes/d1/projects/MET/MET_development/MET-develop/met/bin
touch empty_obs.txt
$MET_BIN/ascii2nc empty_obs.txt empty_obs.nc -format met_point
$MET_BIN/point2grid empty_obs.nc G211 empty_grid.nc -field 'name="Fscale";
level="";'
OR 
$MET_BIN/point2grid empty_obs.nc G211 empty_grid.nc -config Point2GridConfig_grib -field 'name="Fscale";
level="";'

Add following three lines to Point2GridConfig_grib
var_name_map = [
   { key = "Fscale";    val = "Fscale"; }          // GRIB: 
];

• Do these changes include sufficient documentation and testing updates? [Yes]

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

Pull Request Checklist

See the METplus Workflow for details.

• Complete the PR definition above.
• Ensure the PR title matches the feature or bugfix branch name.
• Define the PR metadata, as permissions allow.
  Select: Reviewer(s), Project(s), and Milestone
• After submitting the PR, select Linked Issues with the original issue number.
• After the PR is approved, merge your changes. If permissions do not allow this, request that the reviewer do the merge.
• Close the linked issue and delete your feature or bugfix branch from GitHub.

The fix is changing error message to warning message and continue instead of stopping if the input data is empty. If empty input (the number of header is 0 and the number of obs data is1, skip reading header variables and obs variables.

Warning message instead of an error message:

$MET_BIN/point2grid empty_obs.nc G211 empty_grid.nc -field 'name="Fscale"; level="";' -v 4

WARNING:
WARNING: process_point_file() -> Invalid GRIB code [Fscale]
WARNING:        But ignored because of empty input

[JohnHalleyGotway]

Changes look fine to me. I like having triggering the warning instead of error when the input file has 0 observations. I ran the tests recommended and see the behavior that Howard describes. Howard, please go ahead and merge these changes into develop.