DPPT-970: Add Telescope Model sources and codify scan type beams

Make two changes to the SDP AssignResources schema.

The first is to add the "telmodel_sources" key to the execution block to specify the Telescope Model data sources that should be used in the processing. This has been implemented as a list.

The second is to codify the existing keys in the "beams" entry in the scan types and add the "integration_time" key. The "scan_types" part of the execution block looks like this (ignoring other keys):

{
  "execution_block": {
    "scan_types": [
      {
        "scan_type_id": "...",
        "beams": {
          "vis0": {
            "field_id": "...",
            "channels_id": "...",
            "polarisations_id": "...",
            "integration_time": 0.14
          }
        }
      }
    ]
  }
}

I have also taken the opportunity to remedy an omission from the documentation. The "max_length" key in the execution block, which sets its maximum length in seconds, was described in AssignResources 0.3. It unfortunately lost its description in version 0.4 onwards, possibly because the schema was rearranged. I have added the description for the more recent versions.