Units Test 20: repeated unknown units are not normalized

This is probably a bug in das2C, but the unit test for das2D is producing the error:
```
ERROR: Test 20 Failed, repeated unknown units are not normalized to first instance, hertz kiloelectronvolt**-1 centimeters**-2 sterradian**-1 != cm**-2 keV**-1 s**-1 sr**-1
```
when run against the version of das2C compiled for the Voyager MAG data reader.
