Only use the yc=0 measurements for LV1

* proplyd-shell-measurements.org:
Export proplyd shell parameters to data files

* proplyd-shell-measurements.org:  The files are exported from org-mode
  tables to the following two data files.  They are then read in by
  rc-r0-rotated.py

* best-proplyds.dat:  These are the symmetric and unproblematic proplyds

* second-proplyds.dat:  These are either very asymmetric or otherwise
  problematic.
Better way of calculating the every15 mask
Add error bars to Rc/R0 vs R0 plot

* rc-r0-rotated.py:  Also use the new measurements and adjust the
  vertical range.  There is a bizarre issue with bogus items turning up
  in the legend.  This only happens when plt.errorbars is used.

* proplyd-shell-R0-Rc-errors.pdf:  This is the new figure file
Refactor Rc-R0 into separate programs

* rc-r0-rotated-plot.py:  New program to do solely the graphics
* rc-r0-rotated.py:  This now just does the calculation and saves the
  curves as a JSON file