Add support for new point_weight_flag to the Point-Stat and Ensemble-Stat tools

Describe the New Feature

The MET Grid-Stat tool supports a configuration option named grid_weight_flag to define weights for computing statistics aggregated over multiple grid points. The grid weighting is based on the area each grid point represents, giving larger weight to grid boxes with larger areas.

This task is to develop a method for weighting the aggregation of point observations. And the same basic motivation applies, wanting to avoid overemphasizing areas with dense observations, and underemphasizing areas with sparse observations.

Consider also whether point_weight_flag should be added to Stat-Analysis jobs when aggregating MPR lines to compute partial sums, contingency tables, and statistics.

This request originally arose when aggregating SEEPS for individual stations into a spatial summary. The UK Met Office defines weights for that aggregation based on the spatial density of those stations. However, those weights are pre-defined and static since the stations they use are consistent run-to-run.

Recommend that when implementing this in MET, the weights NOT be static, in general. Instead, dynamically compute them for each verification task based on the location of the observations being processed.. The number and location of point observations can change dramatically run-to-run based on the masking region, variable, and data source. That being said, re-defining them in each run would likely be slower. Recommend that when specifying a mask.sid list of station id's we provide an option to define a fixed station weight.

The tasks for this issue include:

Collaborating with @RachelNorth and @mpm-meto to clarify the algorithm for defining these weights.
Add point_weight_flag configuration option for each verification task in Point-Stat and Ensemble-Stat with a default value of NONE, meaning apply a weight of 1 to all points.
Support setting point_weight_flag equal to DENSITY to define the weights on the fly based on station location.
When mask.sid is set to a file and station names are read from that file, add an option for a raw weight to be specified for that station (perhaps SID(weight) with a numeric weight?). When aggregating across multiple stations, note that the true weight should be computed as the raw station weight divided by the sum of the weights of all points.
Set the existing weights in the PairDataPoint and PairDataEnsemble classes and ensure that those weights are used correctly in the computation of statistics.
In particular, ensure that the these weights are used in the aggregation of SEEPS_MPR data to compute the aggregated SEEPS data.

Acceptance Testing

List input data types and sources. Describe tests required for new functionality.

Time Estimate

1 week.

Sub-Issues

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Define Related Issue(s)

Consider the impact to the other METplus components.

[x] METplus, MET, METdataio, METviewer, METexpress, METcalcpy, METplotpy
[ ] Need a METplus issue to handle configuring the new point_weight_flag option.
[ ] If changes to any output line types are made, would need a METdataio issue to handle those changes.

New Feature Checklist

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dtcenter / MET