Make endpoint names more descriptive

[nmdefries]

• pub_covid_hosp_facility_lookup arg fips_code name doesn't parallel arg zip. Neither should have "code" in their names.
• pub_gft name too short and not descriptive. Ditto pvt_ght, and pvt_cdc and pub_delphi (although for the last two it's more that "CDC" and "Delphi" are very broad)
• name covidcast_epidata is not informative. What about "covidcast_signal_explorer"?