[BUG] numpy.float64 not supported in Piece / Piecewise

[JonaGL]

Version Confirmation

• I have confirmed this bug exists on the latest release of FlixOpt

Bug Description

Creating an object of the class Piece in flixOpt by using attributes of the type numpy.float64 throws an error. Converting it to the type float before solves the issue.

Minimal Reproducible Example

# in the Complex example.py 5 was changed with np.float64(5) to get the unsupported type

    piecewise_conversion = fx.PiecewiseConversion(
        {
            P_el.label: fx.Piecewise([fx.Piece(np.float64(5), 30), fx.Piece(40, 60)]),
            Q_th.label: fx.Piecewise([fx.Piece(6, 35), fx.Piece(45, 100)]),
            Q_fu.label: fx.Piecewise([fx.Piece(12, 70), fx.Piece(90, 200)]),
        }
    )

Error Output

flixopt.core.ConversionError: Converting data <class 'numpy.float64'> to xarray.Dataset raised an error: coords is not dict-like, but it has 1 items, which does not match the 0 dimensions of the data

[FBumann]

Combine with #397

[FBumann]

#397