Scatter plots on maps without an associated data variable.

[rhoneyager-tomorrow]

Description

emcpy supports creating a scatter plot on a map without an associated data variable. This PR makes the small changes to eva needed to work with this feature.

This is quite useful when plotting observation locations. Here's an example, based on this PR, where I can easily highlight duplicate obs data locations in my SSMIS data feed.

Transform to generate variable:

transforms:
  - transform: accept where
    new name: SSMIS::Ascending_Orbital_Node_Highlighting_Duplicates::latitude
    starting field: SSMIS::MetaData::latitude
    where:
      - SSMIS::MetaData::satelliteAscendingFlag == 0
      - SSMIS::MetaData::duplicateDataCheck == 1
  - transform: accept where
    new name: SSMIS::Ascending_Orbital_Node_Highlighting_Duplicates::longitude
    starting field: SSMIS::MetaData::longitude
    where:
      - SSMIS::MetaData::satelliteAscendingFlag == 0
      - SSMIS::MetaData::duplicateDataCheck == 1

Plotting YAML section:

        - type: MapScatter
          longitude:
            variable: ${variable}::Ascending_Orbital_Node_Highlighting_Duplicates::longitude
          latitude:
            variable: ${variable}::Ascending_Orbital_Node_Highlighting_Duplicates::latitude
          markersize: 2
          label: Duplicate Observations
          color: 'red'

Dependencies

None

Impact

None

[CoryMartin-NOAA]

Looks good from what I can tell, thanks @rhoneyager-tomorrow !