Allow heights in modelchains weather_df multiindex to be strings #86
Description
The weather_dfused in the ModelChain is required to be a multiindex dataframe in order to allow variables to be provided at different heights. At the ongoing developer meeting we decided to leave it this way but make its usage more convenient by allowing the heights to be provided as strings which occurs when a multiindex dataframe is read from a csv. @uvchik volunteered to implement it.