generalize the sediment parcel start location

[amoodie]

generalize the sediment parcel start location from fixed along back wall to any place that parcels can start at. Simple unravels the flat index of the start, rather than assuming it is always at x=0 and the index is only the y index.

This is a nonbreaking change that will be invisible to all users.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.52%. Comparing base (f747492) to head (538873c).
⚠️ Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
pyDeltaRCM/sed_tools.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #297   +/-   ##
========================================
  Coverage    78.52%   78.52%           
========================================
  Files           12       12           
  Lines         2673     2673           
========================================
  Hits          2099     2099           
  Misses         574      574           

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