fix addition of DerivativeCoefficientRows with different Start values

[Dougal-s]

Adding together two DerivativeCoefficientRow variables with different Start values throws a bounds error.
For example,

import SummationByPartsOperators
row1 = SummationByPartsOperators.DerivativeCoefficientRow{Int, 0, 3}([1,2,3])
row2 = SummationByPartsOperators.DerivativeCoefficientRow{Int, 1, 3}([1,2,3])
row1 + row2

results in

ERROR: BoundsError: attempt to access NTuple{4, Int64} at index [0]
Stacktrace:
 [1] getindex
   @ ./tuple.jl:31 [inlined]
 [2] getindex
   @ ~/.julia/packages/StaticArrays/EHHaF/src/SArray.jl:62 [inlined]
 [3] +(coef_row1::SummationByPartsOperators.DerivativeCoefficientRow{Int64, 0, 3}, coef_row2::SummationByPartsOperators.DerivativeCoefficientRow{Int64, 1, 3})
   @ SummationByPartsOperators ~/.julia/packages/SummationByPartsOperators/DBpfT/src/SBP_operators.jl:149
 [4] top-level scope
   @ REPL[4]:1

After applying the fix, it returns

SummationByPartsOperators.DerivativeCoefficientRow{Int64, 0, 4}([1, 3, 5, 3])

[ranocha]

Thanks for fixing this. Could you please add some unit tests as well to keep track of this?

[Dougal-s]

Sure thing! I wasn't sure where to put the unit tests, so I added them to the end of linear_combinations_of_operators_test.jl. Should I put them into a separate file instead?

[ranocha]

Thanks a lot!

[ranocha]

Sure thing! I wasn't sure where to put the unit tests, so I added them to the end of linear_combinations_of_operators_test.jl. Should I put them into a separate file instead?

A new file would be fine 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.70%. Comparing base (b98874b) to head (5178458).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #260   +/-   ##
=======================================
  Coverage   90.70%   90.70%           
=======================================
  Files          33       33           
  Lines        5112     5112           
=======================================
  Hits         4637     4637           
  Misses        475      475           

Flag	Coverage Δ
unittests	90.70% <100.00%> (ø)

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[coveralls]

coverage: 91.609%. remained the same
when pulling 5178458 on Dougal-s:main
into b98874b on ranocha:main.