Fixed two wrong results in tests for addition of offset units
Dropped creation of delta_dimension and delta_units for units without offset

The dimension delta_temperature was dropped because having two dimensions
for temperature has no benefit but only complicates the implementation of
calculations with quantities that have an offset unit. For temperature,
delta_kelvin and delta_degR do no longer exist. There are now only two
delta_units left, delta_degC and delta_degF, which are also of dimension
"temperature".
Implemented correct addition and subtraction for offset units

All tests for addition and subtraction for quantities with offset units pass.
Other (older) tests partly fail and need to be cleaned up: "delta_kelvin",
"delta_degR" and the dimension "delta_temperature" do no longer exist.