Support additional types of custom properties in the Cesium map config

[robyngit]

Since commit 451d93e, custom properties can be configured for Map Assets in a Cesium Map. Custom properties are properties that are not stored in geospatial features, but may be created from existing properties. These custom properties can be used to conditionally hide features, color-code a layer, or in the feature info template.

For example, imagine that features in a certain vector layer have a "timestamp" property. Timestamps are given in the format 2018-08-16T19:20:30.45+01:00, but it's preferred that the date is displayed in the FeatureInfoView in the friendlier format Thursday, August 16, 2018. This property can be created by adding customProperties to to the configuration for this layer, e.g.:

"layers": [
  {
    "label": "Ice Wedge Polygons",
    "type": "Cesium3DTileset",
    ...
    "customProperties": {
      "friendlyDate": {
        "type": "date",
        "property": "timestamp",
        "format": "dddd, MMMM D, YYYY"
      }
    }
  }
]

The new friendlyDate property can then be referenced like a regular property in any other part of the configuration (featureTemplate, colorPalette, vectorFilters).

So far, the only custom properties that are supported are dates and strings, and the "string" type only allows a static string (i.e. it cannot include other, existing properties.) This goal of this issue is to extend support for custom properties to include:

• Number formatting (e.g. number of significant digits)
• Creation of boolean properties based on other properties (e.g. area > 10 -> true)
• Dynamic strings that can include multiple property values, (e.g. "The area of " + name + " is " + area + " km2") and/or can be partial matches of existing string properties (perhaps using regular expressions)

Note: Shell functions to support these new custom properties exist here, and should be called from here. Each newly supported custom property can be documented here