The spatial calibration data are usually delivered as an Excel spreadsheet
named P006DeltaVectors.xlsx.  The updates for new dates are added as
separate tables below the existing data.

The pipeline requires the linear coefficients of the x and y offset of the
secondary array from the primary array (the red offset relative to the
blue), as a function of the INDPOS, for each dichroic setting.  It gets
used as:
  primary_offset_x = bx + ax * indpos_pri - RxCx
plus (if necessary):
  secondary_offset_x = (s_bx - bx) + (s_ax * indpos_sec - ax * indpos_pri)
and similar for the offsets in y.  The ax, bx, RxCx, ay, bx, and RxCy
coefficients for each dichroic (D105 and D130) and camera (RED and BLUE)
are stored in the notebook as a, b, U under the x column
and a, b, V under the y column, respectively.

To make something the pipeline understands:
1. Save the spreadsheet to a new name in this directory (data/spatial_cal),
   indicating the current date.  For example, P006DeltaVectors_20170221.xlsx
   for a file received February 21, 2017.
2. Open the Excel file and find the Data tab.
3. Identify the a, b, and U/V numbers in the secion for the new data, at
   the bottom of the sheet.
4. Open the FIFI_LS_DeltaVector_Coeffs.txt file and add four lines for the new
   data.  The lines should start with:
      # yymm
      99999999  b  105   
      99999999  b  130   
      99999999  r  105  
      99999999  r  130  
   where yymm is a comment with the new date provided in the Excel notebook,
   b indicates the BLUE camera, r indicates the RED camera and 105/130
   indicate the dichroic setting.
5. Copy the relevant coefficients from the notebook into each of the new lines
   in the text file, in the order b_x, a_x, U, b_y, a_y, V.
6. In the text file, change the end date for the previous series from
   99999999 to a date beyond the end of the previous series, and before the
   beginning of the next series.  For example, if a series ended with a flight
   on 20151104, and the next series does not start until February 2016, the date
   can be set to 20151231.
7. If you receive new pixel calibrations, save them in this directory
   (data/spatial_cal).
8. In poscal_defaults.txt, change the 99999999 date (current configuration)
   to a date beyond the end of the previous series, and before the beginning
   of the next series.
9. Add the new poscal files to poscal_defaults.txt in a similar manner to
   step 4, in the form:
   # date 
   99999999 r spatial_cal/pixel_pos_red_20150828.txt