Adding UCIWWEIHR model without WW.

[cbernalz]

Goal

Add a way to forecast and nowcast using the UCIWWEIHR model without the wastewater data.

Requirements

• Add a model function that does not require wastewater data using multiple dispatch.
• Add a fit function that does not require wastewater data using multiple dispatch.
• Add a gq_pp function that does not require wastewater data using multiple dispatch.