Add extension for EnergyModelsGUI for variable descriptions

[Zetison]

To get rid of warnings when running the graphical user interface using EnergyModelsGUI together with EnergyModelsHeat and to provide descriptive names to the variables in the GUI you should add a file descriptive_names.yml in a folder called EMGUIExt which must be located in a folder ext of the EnergyModelsHeat repository. As only TimeProfiles (and JuMP variables with a time dimension) are plotted, only these fields need provided descriptions. The descriptive_names.yml looks something like

# This file contains descriptions of EnergyModelsHeat-structures and variables 
# with fields of type TimeStruct.TimeProfile
structures:
  HeatPump:
    cap:                    "Installed capacity"
    t_source:               "Temperature profile of the heat source"
    t_sink:                 "Sink temperature of the condensator"
    eff_carnot:             "Carnot Efficiency"
    opex_var:               "Variable operating expense per energy unit produced"
    opex_fixed:             "Fixed operating expense per installed capacity"
  ResourceHeat:
    t_supply:               "Supply temperature in °C"
    t_return:               "Return temperature in °C"
  DHPipe:
    cap:                    "Heat transport capacity of the pipe"
    t_ground:               "Ground temperature in °C"
  HeatExchanger:
    cap:                    "Installed capacity"
    opex_var:               "Variable operating expense per energy unit produced"
    opex_fixed:             "Fixed operating expense"
  DirectHeatUpgrade:
    cap:                    "Installed capacity"
    opex_var:               "Variable operating expense per energy unit produced"
    opex_fixed:             "Fixed operating expense"

# Add description for all JuMP variables using a TimeStruct.TimeProfile type
variables:
  dh_pipe_loss:             "Heat losses in DH pipes"

Note that this requires EnergyModelsGUI v0.5.11 or higher.

[JulStraus]

Solved through #20