FutureWarning: Adjust float conversion to resolve deprecation warning in heat_building.py

[Stefanie08]

During the run of the function get_sigmoid_parameters() in heating_building.py the following FutureWarning is raised:

FutureWarning: Calling float on a single element Series is deprecated and will raise a TypeError in the future. Use float(ser.iloc[0]) instead

The warning concerns the script between lines 271-275.

        a = float(sigmoid["parameter_a"])
        b = float(sigmoid["parameter_b"])
        c = float(sigmoid["parameter_c"])
        if self.ww_incl:
            d = float(sigmoid["parameter_d"])

I ran this script through the oemof-B3 model with a Windows 10 operating system and Python 3.10.12
This could be adapted to avoid TypeErrors.

[ddceruti]

My latest pull request addresses this issue directly #55.