Use output_reference_idx as the default for creating compressed slc…

…s with `ALWAYS_FIRST` (#465)

Sara found the problem with repeatedly running disp-s1 with a reference
update:

ministack 0:
compressed_t042_088905_iw1_20160810_20160810_20170407.h5
ministack1:
compressed_t042_088905_iw1_20170817_20170419_20171028.h5
ministack2:
compressed_t042_088905_iw1_20160810_20171109_20180508.tif

Even though in Run2 we are making interferograms relative to
20170817, the `compressed_reference_idx` snapped back to 0.

src/dolphin/stack.py

@@ -468,7 +468,9 @@ def plan(
             elif self.compressed_slc_plan == CompressedSlcPlan.ALWAYS_FIRST:
                 # Simplest operational version: CompSLCs have same base phase,
                 # but different "residual" added on
-                compressed_reference_idx = 0
+                # We use the `output_reference_idx`, 0 by default, but this index
+                # may be passed in if we are manually specifying an output
+                compressed_reference_idx = self.output_reference_idx
             elif self.compressed_slc_plan == CompressedSlcPlan.FIRST_PER_MINISTACK:
                 # Like Ansari, 2017 paper: each ministack is "self contained"
                 compressed_reference_idx = num_ccslc