Realistic Measurement sampling errors

[nstarman]

Description

Subclass MeasurementErrorSampler to make a InterpolatedGridMeasurementErrorSampler that takes a catalog of data points, for example Gaia, takes the errors and creates a 3D interpolated & smoothed grid of the errors as a function of the position. This can be evaluated with the MeasurementErrorSampler given positions from the PotentialSampler class to create realistic measurement errors.

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