Constructor for lognormal distribution that takes average and variance

[sbenthall]

See #519 and #611

In ConsPortfolioModel (and possibly elsewhere), there are places where there is duplicate code computing a Log Normal mu and sigma from a given average and variance.

https://github.com/econ-ark/HARK/blob/master/HARK/ConsumptionSaving/ConsPortfolioModel.py#L243-L261

Rather than doing this twice in the model code, it would be better to have an alternative constructor for the lognormal distribution that took these values directly.

[sbenthall]

Also here:
https://github.com/econ-ark/HARK/blob/master/HARK/ConsumptionSaving/ConsPortfolioModel.py#L243-L261