Add interface to event portal filtered queries

This is to propose adding a new method in api.py to request filtered queries like explained here: https://www.gw-openscience.org/apidocs/#event5. And also a higher level method in datasets to do basically the same, but just returning the names.

Proposed API:

• api.fetch_filtered_events_json(select, host=DEFAULT_URL)
• datasets.query_events(select)

Example:

• api.fetch_filtered_events_json

  >>> from gwosc import api
  >>> api.fetch_filtered_events_json(select=["mass-1-source < 5", "mass-2-source < 10", "luminosity-distance in (10, 100)"])
  {
    "events": {
      "GW170817-v3": {
        "GPS": 1187008882.4,
        "catalog.shortName": "GWTC-1-confident",
        "chi_eff": 0.0,
        "chi_eff_lower": -0.01,
        "chi_eff_unit": "",
        ...
        "version": 3
      }
    }
  }

  This will internally call /eventapi/json/query/show?max-mass-1-source=5&max-mass-2-source=10&min-luminosity-distance=10&max-luminosity-distance=100

• datasets.query_events

  >>> from gwosc import datasets
  >>> datasets.query_events.(select=["mass-1-source < 5", "mass-2-source < 10", "luminosity-distance in (10, 100)"])
  ["GW170817-v3"]

  This will internally call /eventapi/json/query/show?max-mass-1-source=5&max-mass-2-source=10&min-luminosity-distance=10&max-luminosity-distance=100

---

This borrows the syntax used in GWpy.EventTable for filtering, with the idea that eventually, the filtering that EventTable does, can be done on gwosc.org's backend.