xarray implementation of sampling in vaccine dosage model

[kellypierce]

https://github.com/eho-tacc/episimlab/blob/50024afbb1dc6f185dd8bf3408f623a3e8f735ed/episimlab/models/vaccine.py#L112

The current vaccine dosage calculation requires conversion from xarray -> numpy array -> xarray. There is probably a smarter way to do this.

Constraints:

• must be able to check that parameters for np.random.hypergeometric() are valid
• must be able specify age and risk group dosage allocations, and support an allocation of zero doses for some groups