component_electrolyzer_waste_heat: Zeros have to be removed from the piecewise linear heat characteristic

[ulrikeherrmann]

Usage of the component yields pyomo warning:
**WARNING: Piecewise component 'PiecewiseLinearTransformerBlock.piecewise[("<oemof.solph.custom.PiecewiseLinearTransformer: 'this_ely_thermal'>", 0)]' has detected slopes of consecutive piecewise segments to be within 1e-08 of one another. Refer to the Piecewise help documentation for information on how to disable this warning.

For the waste heat characteristic, currently the same breakpoints are used as for the hydrogen characteristic. Since the waste heat is zero if the temperature is below the max temperature, there are several breakpoints with value zero. These should be removed afterwards.