Consultation about boiler steam duty

Hello, the ```duty_over_mol = 39000``` (https://github.com/BioSTEAMDevelopmentGroup/biosteam/blob/master/biosteam/facilities/_boiler_turbogenerator.py#L397) set in the boiler source code seems to only apply to low pressure steam. If medium pressure steam and high pressure steam are also required in the process, the steam flow calculated by ```duty_over_mol = 39000``` seems to be wrong. This seems to affect the calculation of boiler costs and the flow rate calculation of boiler-related stream. Thanks for your help.

Start by reading biosteam/facilities/_boiler_turbogenerator.py around line 397, where duty_over_mol is set to 39000. Check how medium- and high-pressure steam requirements affect the boiler cost and boiler-related stream flow calculations. Done means determining whether the current calculation is valid for those pressures and documenting or correcting the affected behavior.

Written by the indexing model from the issue text.