Update priors for model calibration.

[FrankErrickson]

The current prior distributions and descriptive statistics are hard-coded. This can lead to bugs if users want to do a sensitivity analysis with different priors and have to modify the source code. This should be modified so users can:

1. Specify a certain prior distribution type using Distributions.jl for each model parameter.
2. Specify the statistical parameters for each prior distribution.
3. Alternatively, we could specify upper/lower bounds and then fit statistical parameters to a given distribution.
4. Overall, this should probably be either in a priors.jl file that users can modify, or an option to change from a main.jl file.