add heat equation tutorial

[ranocha]

I added another tutorial on the heat equation to also demonstrate the usage of Tridiagonal matrices (for homogeneous Neumann BCs).

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All modified and coverable lines are covered by tests ✅

Project coverage is 98.09%. Comparing base (258c798) to head (5117ee0).

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@@           Coverage Diff           @@
##             main     #105   +/-   ##
=======================================
  Coverage   98.09%   98.09%           
=======================================
  Files           6        6           
  Lines        1415     1415           
=======================================
  Hits         1388     1388           
  Misses         27       27           

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[SKopecz]

Thanks! I have some minor comments. But we should really use $u_0\geq 0$ here.

[ranocha]

Thanks! I have added another tutorial on the heat equation with homogeneous Dirichlet BCs.

[ranocha]

I have also bumped the version so that we can tag a new release before merging our breaking change from #104

[SKopecz]

Great, thanks!