Set proper timestamps in residual calculated absolutes

Fixes #75

When geomagio.residual.calculate() is called with adjust_reference set equal
to True, the start/end timestamps now both reflect the time of the first absolute
measurement (i.e., West Down). This is consistent with the residual absolute
spreadsheet calculations.

geomagio/residual/Calculation.py

@@ -95,6 +95,12 @@ def calculate_D_absolute(
     D Absolute
     """
     mean = average_measurement(measurements, DECLINATION_TYPES)
+    if reference:
+        starttime = reference.time
+        endtime = reference.time
+    else:
+        starttime = mean.time
+        endtime = mean.endtime
     reference = reference or mean
     # average mark
     average_mark = average_measurement(measurements, MARK_TYPES)
@@ -136,8 +142,8 @@ def calculate_D_absolute(
             absolute=d_abs,
             baseline=d_b,
             shift=shift,
-            starttime=mean.time,
-            endtime=mean.endtime,
+            starttime=starttime,
+            endtime=endtime,
         ),
         meridian,
     )
@@ -173,21 +179,26 @@ def calculate_HZ_absolutes(
     if reference:
         h_abs = np.sqrt((h_b + reference.h) ** 2 + (reference.e) ** 2)
         z_abs = z_b + reference.z
+        starttime = reference.time
+        endtime = reference.time
+    else:
+        starttime = mean.time
+        endtime = mean.endtime
     # return absolutes
     return (
         Absolute(
             element="H",
             baseline=h_b,
             absolute=h_abs,
-            starttime=mean.time,
-            endtime=mean.endtime,
+            starttime=starttime,
+            endtime=endtime,
         ),
         Absolute(
             element="Z",
             baseline=z_b,
             absolute=z_abs,
-            starttime=mean.time,
-            endtime=mean.endtime,
+            starttime=starttime,
+            endtime=endtime,
         ),
     )
 
@@ -279,5 +290,5 @@ def calculate_scale_value(
         / corrected_f
     ) + (m2.angle - m1.angle)
     residual_change = m2.residual - m1.residual
-    scale_value = corrected_f * field_change / np.abs(residual_change)
+    scale_value = np.radians(corrected_f * field_change / np.abs(residual_change))
     return scale_value