Always fetch missing CellML files

[kwabenantim]

Description
FetchContent currently skips downloading CellML files if configuration has already been done once. This fix was added in #26 and prevents FetchContent from throwing errors when it can't overwrite existing files, which can happen when cmake is called twice or ccmake is used.

The current behaviour means that CellML files which have been accidentally removed after the inital configuration will not be replaced when cmake is called again.

Steps To Reproduce

cd /path/to/Chaste/build
cmake ..
rm /path/to/Chaste/projects/ApPredict/src/cellml
cmake ..
make ApPredict

Error

[ 82%] Building CXX object projects/ApPredict/CMakeFiles/chaste_project_ApPredict.dir/src/stats/BayesianInferer.cpp.o
/home/runner/_work/ApPredict/ApPredict/Chaste/projects/ApPredict/src/single_cell/CipaQNetCalculator.cpp:42:10: fatal error: ohara_rudy_cipa_v1_2017Cvode.hpp: No such file or directory
   42 | #include "ohara_rudy_cipa_v1_2017Cvode.hpp"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[3]: *** [projects/ApPredict/CMakeFiles/chaste_project_ApPredict.dir/build.make:258: projects/ApPredict/CMakeFiles/chaste_project_ApPredict.dir/src/single_cell/CipaQNetCalculator.cpp.o] Error 1
make[3]: *** Waiting for unfinished jobs....
[ 82%] Building CXX object projects/ApPredict/CMakeFiles/chaste_project_ApPredict.dir/src/stats/LinearDiscriminantAnalysis.cpp.o
make[2]: *** [CMakeFiles/Makefile2:22324: projects/ApPredict/CMakeFiles/chaste_project_ApPredict.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:2327: CMakeFiles/project_ApPredict.dir/rule] Error 2
make: *** [Makefile:314: project_ApPredict] Error 2

To Do

• Try to force overwrites (delete and re-fetch), or check for missing files and fetch only those.