Add support for partMode explore

[jhoobergs]

• See https://docs.numbas.org.uk/en/latest/question/explore.html#term-maximum-mark
• Adds following fields to ExamQuestion: (with a flattened ExamQuestionExploreData)
  • partsMode ("all" vs "explore"
  • maxMarks (0 means -> just sum of limit of objectives, otherwise -> reduce to this amount if more points would be awarded based on scores and penalties)
  • objectives (list of { name, limit (string of number?) and mode (currently only sum?))
  • penalties: list
  • objectiveVisibility
  • penaltyVisibility

"partsMode": "explore",
"maxMarks": 0,
"objectives": [
  { "name": "How Numbas works", "limit": "8", "mode": "sum" },
  {
    "name": "Mathematical expressions",
    "limit": "16",
    "mode": "sum"
  },
  { "name": "Numbers", "limit": "8", "mode": "sum" }
],
"penalties": [],
"objectiveVisibility": "always",
"penaltyVisibility": "always",

• Add nextParts, suggestGoingBack and exploreObjective to part:

"nextParts": [
  {
    "label": "How Numbas works",
    "rawLabel": "",
    "otherPart": 1,
    "variableReplacements": [],
    "availabilityCondition": "",
    "penalty": "",
    "penaltyAmount": 0,
    "lockAfterLeaving": false
  },
  {
    "label": "Mathematical expressions",
    "rawLabel": "",
    "otherPart": 10,
    "variableReplacements": [],
    "availabilityCondition": "",
    "penalty": "",
    "penaltyAmount": 0,
    "lockAfterLeaving": false
  },
  {
    "label": "Numbers",
    "rawLabel": "",
    "otherPart": 33,
    "variableReplacements": [],
    "availabilityCondition": "",
    "penalty": "",
    "penaltyAmount": 0,
    "lockAfterLeaving": false
  }
],
"suggestGoingBack": false,
"exploreObjective": null, // This is a string with the name of the objective