Remove auto-sift, allow manual shift in calculate_D_absolute

Turns out there are operational limitations that don't allow a
deterministic removal of the 180 degree ambiguity that exists
for calculated Declination. The one previously implemented only
worked for a subset of observatories. Now, we force all calculated
Declinations to point north(ish), which  is valid for most of the
globe, and all of the USGS observatories. However, if we ever
take absolute measurements in a location where the local magnetic
field points south(ish), we can simply provide a "shift" of +/-
180 degrees.