Add eshift to restart file to preserve continuity

[danielhollas]

TODO:

• Fix tests
• Make the restart more robust, print helpful error for old versions of restart files

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.04%. Comparing base (85cf84f) to head (577e234).

Files with missing lines	Patch %	Lines
src/sh_integ.F90	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
- Coverage   84.06%   84.04%   -0.03%     
==========================================
  Files          47       47              
  Lines        6602     6606       +4     
  Branches      764      765       +1     
==========================================
+ Hits         5550     5552       +2     
- Misses        853      854       +1     
- Partials      199      200       +1     

Flag	Coverage Δ
unittests	18.39% <0.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/surfacehop.F90	89.15% <100.00%> (ø)
src/sh_integ.F90	91.05% <66.66%> (-0.65%)	⬇️

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