Updated Heaviside definition to avoid issues with underflow/NaNs

Current behavior:

julia> Heaviside_smooth(Forward_Diff.Dual(-10.0, 0.0))
Dual{Nothing}(0.0,NaN)

Expected/corrected behavior:

julia> Heaviside_smooth(Forward_Diff.Dual(-10.0, 0.0))
Dual{Nothing}(0.0,0.0)