Support multiple returnPeriods/IMLs in DisaggCalc

@pmpowers from our discussion during the NSHM Code Club on Feb 3rd, is it possible/feasible to implement multiple DisaggCalcs within the same DisaggCalc call, i.e. produce disaggregation data for multiple return periods or multiple intensity measure levels? I suppose the way it might be specified by the user is in the config.json, where the 'disagg' key could allow the user to specify return periods as a list, e.g.

"disagg": {
    "returnPeriod": [475,975,2475],
    "bins": {
      "rMin": 0.0,
      "rMax": 1000.0,
    }, 

or to specify intensity measure levels as a list:

"disagg": {
    "imls": [0.01,0.1,0.5,1.0],
    "bins": {
      "rMin": 0.0,
      "rMax": 1000.0,
    },