Taal: add trixi_include and move definitions of abstract types for dispatch

[ranocha]

This resolves some comments left in #187.

[codecov]

Codecov Report

Merging #202 into dev will decrease coverage by 0.01%.
The diff coverage is 47.61%.

@@            Coverage Diff             @@
##              dev     #202      +/-   ##
==========================================
- Coverage   89.24%   89.22%   -0.02%     
==========================================
  Files          52       52              
  Lines        9923     9935      +12     
==========================================
+ Hits         8856     8865       +9     
- Misses       1067     1070       +3     

Impacted Files	Coverage Δ
src/Trixi.jl	100.00% <ø> (ø)
src/callbacks/alive.jl	82.60% <ø> (ø)
src/equations/equations.jl	81.63% <ø> (ø)
src/semidiscretization.jl	60.00% <ø> (ø)
src/solvers/dg/dg.jl	68.79% <0.00%> (ø)
src/solvers/dg/dg_2d.jl	85.03% <0.00%> (ø)
src/run.jl	93.63% <75.00%> (-0.88%)	⬇️
src/auxiliary/auxiliary.jl	86.84% <100.00%> (ø)

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[ranocha]

If this looks good for you, could you please approve it explicitly (using GitHub's review tools)? I think it would be nice if we got used to approving PRs explicitly like that.

[sloede]

If this looks good for you, could you please approve it explicitly (using GitHub's review tools)? I think it would be nice if we got used to approving PRs explicitly like that.

I strongly concur!

[ranocha]

Thanks!