add support for getting hospitalizations data

[elray1]

to start, only support US states and national, getting data from the covidcast API.

For states:

• rename jhu to something more general
• add argument to jhu.load_us to specify which data source to load from (jhu or covidcast)
• add a utility function similar to load_us_time_series that gets cases, hospitalizations, or deaths data from covidcast for all states. Function arguments are target_variable ("cases", "hospitalizations", or "deaths"), and an optional as_of parameter. It should return a pandas data frame with the date as an index and one column for each state-level location. The column names should match the location names returned by jhu.get_state_info(). The values in the columns will be the case/hospitalization/death counts (just one for a single call to the function) in each state on the given date.
• At https://github.com/dsheldon/mechbayes/blob/c51d09038c040d1a18baa8b6da595aa6b4e6fad8/mechbayes/jhu.py#L145-L146 , condition on the data source to either do the existing calls to load_us_time_series or call the new function above. For now, when calling the utility function for covidcast, just omit the as_of parameter to get the latest available data, matching current behavior for cases and deaths. If data source is covidcast, add hosps to the concat call a couple lines down.

For US national:

• add an entry to the dictionary at https://github.com/dsheldon/mechbayes/blob/c51d09038c040d1a18baa8b6da595aa6b4e6fad8/mechbayes/jhu.py#L26 for hosp. For now, it should have observed value for the US only, and nan's for all other locations

For counties:

• we don't have observations for counties, add a column with all nan's

[dsheldon]

This proposal looks pretty good; it always takes me some trial and error to get all the pandas commands right.

An alternate way would be load the hospital data separately (not in jhu.py) and add it in the higher-level utils in util.py. E.g., for states, you could add it around here: https://github.com/dsheldon/mechbayes/blob/c51d09038c040d1a18baa8b6da595aa6b4e6fad8/mechbayes/util.py#L58-L69

After running this code, you would need to add a hosp column to data[place]['data'] for each relevant place.

No strong opinion here on which approach is better.