Replace individual max and min values with pairs

[Zarquan]

The current document has lots of maxxxxx and minxxxx properties, using the property names to link them together.

resources:
  compute:
  - name: "compute-001"
    type: "https://www.purl.org/ivoa.net/resource-types/generic-compute"
    spec:
      mincores: 8
      maxcores: 16
      minmemory: 16
      maxmemory: 32 

This means each class object has lots of top level minxxx and maxxxx properties associated with it, cluttering up the class interface.

It is better to define maximum and minimum properties as a re-useable element that contains the min and max values as a pair.

resources:
  compute:
  - name: "compute-001"
    type: "https://www.purl.org/ivoa.net/resource-types/generic-compute"
    spec:
      cores:
        min: 8
        max: 16
      memory:
        min: 16
        max: 32 

It cleans up the class interfaces and enables us to automate some validation that checks that the max value is always greater than the min value.

[Zarquan]

Fixed in #60