Make trixi_include more flexible by allowing a mapping to be passed

[efaulhaber]

This is similar to how Base.include is defined:

include(mod::Module, _path::AbstractString) = _include(identity, mod, _path)
include(mapexpr::Function, mod::Module, _path::AbstractString) = _include(mapexpr, mod, _path)

[coveralls]

Pull Request Test Coverage Report for Build 12471675311

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 95.122%

---

Totals
Change from base Build 12102967779:	0.1%
Covered Lines:	78
Relevant Lines:	82

---

💛 - Coveralls

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.12%. Comparing base (91eb18c) to head (d57665d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   95.00%   95.12%   +0.12%     
==========================================
  Files           5        5              
  Lines          80       82       +2     
==========================================
+ Hits           76       78       +2     
  Misses          4        4              

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[sloede]

Fine by me, though I do not really understand most of the changes 😅. Would be good if @ranocha could give the final green light :-)