Fix the reference for Muparser.

[alarshi]

This PR fixes the old references to muparser library in the documentation.

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• I have read the guidelines in our CONTRIBUTING.md document.

[bangerth]

Yes, nice!

[gassmoeller]

/rebuild

[gassmoeller]

@alarshi all these files are automatically generated from the corresponding .cc files (these are taken from the parameter descriptions in the declare_parameters functions. Could you fix the references in the .cc files? Otherwise these changes will be overwritten the next time someone updates the parameters in the manual.

[bangerth]

@gassmoeller Oh, good catch! I had seen that the changes were in generated files and thought "not necessary, but also no harm", but had totally missed that the origin was left unchanged :-)

[bangerth]

> grep -r 'muparser-format' source/
source/heating_model/function.cc:                                  "muparser library, see Section~\\ref{sec:muparser-format}."
source/prescribed_stokes_solution/function.cc:                                               "muparser library, see Section~\\ref{sec:muparser-format}.")
source/initial_temperature/function.cc:                                              "muparser library, see Section~\\ref{sec:muparser-format}.")
source/initial_temperature/adiabatic.cc:                             "muparser library, see Section~\\ref{sec:muparser-format}.");
source/boundary_heat_flux/function.cc:                                             "muparser library, see Section~\\ref{sec:muparser-format}."
source/boundary_composition/function.cc:                                               "muparser library, see Section~\\ref{sec:muparser-format}.")
source/boundary_velocity/function.cc:                                            "muparser library, see Section~\\ref{sec:muparser-format}."
source/mesh_deformation/function.cc:                                           "muparser library, see Section~\\ref{sec:muparser-format}.")
source/particle/property/function.cc:                                        "muparser library, see Section~\\ref{sec:muparser-format}.")
source/particle/generator/probability_density_function.cc:                                         "Section~\\ref{sec:muparser-format}. The "
source/gravity_model/function.cc:                                  "muparser library, see Section~\\ref{sec:muparser-format}.")
source/initial_composition/function.cc:                                              "muparser library, see Section~\\ref{sec:muparser-format}.")
source/boundary_temperature/function.cc:                                               "muparser library, see Section~\\ref{sec:muparser-format}.")
source/mesh_refinement/minimum_refinement_function.cc:                                              "muparser library, see Section~\\ref{sec:muparser-format}.")
source/mesh_refinement/maximum_refinement_function.cc:                                              "muparser library, see Section~\\ref{sec:muparser-format}.")

[alarshi]

Thanks @bangerth @gassmoeller for the review and explaining the changes! I modified the source files in the latest commit but left the documentation as is.