unit validation using wx_unit

When using quantity properties in schemas, the dimensionality of the unit attribute can be validated using the wx_unit validator.
Similar to wx_shape validation, wx_unit assumes that the object it is attached to has a unit property of type string to validate against.

It is important to note that unit validation does not perform a literal string comparison (like can be archived with enum) but checks the correct dimensionality of the unit.

For example, validation with wx_unit: "m" will be successful against all quantities that represent a length. So this schema

type: object
properties:
  length_prop:
    tag: tag:stsci.edu:asdf/unit/quantity-1.1.0
    wx_unit: "m"

allows all of the following:

length_prop: !unit/quantity-1.1.0 {unit: millimeter, value: 1.5}
length_prop: !unit/quantity-1.1.0 {unit: meter, value: 32}
length_prop: !unit/quantity-1.1.0 {unit: inch, value: 123}

wx_unit is implemented using pint dimensionality-checks. This means that for the unit syntax, every string input that gets picked up by pint correctly can be used. The following are all allowed and result in the same behavior:

wx_unit: "s"
wx_unit: "seconds"
wx_unit: "ms"

It is also possible to validate multiple or nested properties with a single wx_unit entry when using an object-like structure. See this more complex example for details:

tag: "tag:weldx.bam.de:weldx/debug/test_unit_validator-1.0.0"

title: |
  simple demonstration schema for wx_unit validator
type: object
properties:
  length_prop:
    description: |
      a simple length quantity with unit validator
    tag: tag:stsci.edu:asdf/unit/quantity-1.1.0
    wx_unit: "m"

  velocity_prop:
    description: |
      a simple velocity quantity
    allOf:
      - $ref: tag:stsci.edu:asdf/unit/quantity-1.1.0
      - type: object
        wx_unit: "m/s"

  current_prop:
    description: |
      a current quantity of shape [2,2]
    $ref: tag:stsci.edu:asdf/unit/quantity-1.1.0

  nested_prop:
    description: |
      a nested object with two quantities
    type: object
    properties:
      q1:
        description: a nested length of shape [3,3]
        $ref: tag:stsci.edu:asdf/unit/quantity-1.1.0
      q2:
        description: a volume
        $ref: tag:stsci.edu:asdf/unit/quantity-1.1.0
    wx_unit:
      q1: "m"

  simple_prop:
    description: simple property without any references ref
    type: object
    properties:
      value:
        type: number
      unit:
        type: string
    wx_unit: "m"

wx_unit:
  length_prop: "m"
  velocity_prop: "m / s"
  current_prop: A
  nested_prop:
    q2: "m*mm*cm"