Fix BoundaryAdaptedGrid for only one central point

[JoshuaLampert]

This fixes two independent, but related issues regarding the BoundaryAdaptedGrid:

1. Previously the following failed:

D = derivative_operator(MattssonAlmquistVanDerWeide2018Minimal(), 1, 4, 0.0, 1.0, 9)

due to ERROR: ArgumentError: range(0.4999999999999999, stop=0.5, length=1): endpoints differ. This is because there is only one inner point and due to rounding errors the left and right ends are not the same. I simply added an if-statement for this. Let me know if you have a better idea to handle this @ranocha.

2. With this fix, the above code (or, e.g., also D = derivative_operator(MattssonAlmquistVanDerWeide2018Accurate(), 1, 4, 0.0, 1.0, 11) already without the fix above) gives a result, but step(grid) gives zero, again, because there is only one point in the uniform_grid. This results in an operator with mass_matrix equal to zero and Infs and NaNs for the entries of Matrix(D). This modifies the step function for the BoundaryAdaptedGrid to return the actual step also for this edge case.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.59%. Comparing base (560c6d8) to head (cdc5575).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #281   +/-   ##
=======================================
  Coverage   91.58%   91.59%           
=======================================
  Files          35       35           
  Lines        5362     5364    +2     
=======================================
+ Hits         4911     4913    +2     
  Misses        451      451           

Flag	Coverage Δ
unittests	91.59% <100.00%> (+<0.01%)	⬆️

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[coveralls]

coverage: 92.362% (+0.003%) from 92.359%
when pulling cdc5575 on JoshuaLampert:fix-boundaryadaptedgrid
into 560c6d8 on ranocha:main.

[ranocha]

Thanks. I don't have a better idea than an if statement for the special case.

[ranocha]

Could you please bump the version in Project.toml so that I can make a new release directly after merging this?

[JoshuaLampert]

Could you please bump the version in Project.toml so that I can make a new release directly after merging this?

Done in ef87f47.

[JoshuaLampert]

Should I add a test for the case that N - 2M == 1? And if yes, is it enough to test that it successfully creates the operator?

[ranocha]

Adding a test would be great 👍 The test should also cover the second issue that you have described

This results in an operator with mass_matrix equal to zero and Infs and NaNs for the entries of Matrix(D).

[JoshuaLampert]

I added a test in 683dfc9.

[ranocha]

Thanks!