Update the PUML implementation a bit

[davschneller]

Builds upon #19 and includes it.

Refactors more parts of the code, using more arrays/vector etc.

[vikaskurapati]

Just putting it here so that it is linked to this PR,

I tried compiling SeisSol with the version of PUML on this branch in Debug mode, and I got this error on the SuperMUC NG Phase 2 machine.

[ 66%] Building CXX object CMakeFiles/seissol-lib.dir/src/Geometry/PUMLReader.cpp.o
/dss/dsshome1/00/ge26cet3/seissol/src/Geometry/PUMLReader.cpp:485:35: warning: comparison of integers of different signs: 'const int' and 'const std::size_t' (aka 'const unsigned long') [-Wsign-compare]
  485 |       logassert(side >= 0 && side < Cell::NumFaces);
      |                              ~~~~ ^ ~~~~~~~~~~~~~~
/dss/dsshome1/00/ge26cet3/seissol/src/Geometry/PUMLReader.cpp:63:33: note: expanded from macro 'logassert'
   63 | #define logassert(x) logassertI(x, __FILE__, __LINE__)
      |                                 ^
In file included from /dss/dsshome1/00/ge26cet3/seissol/src/Geometry/PUMLReader.cpp:16:
In file included from /dss/dsshome1/00/ge26cet3/seissol/submodules/PUML/Upward.h:24:
/dss/dsshome1/00/ge26cet3/seissol/submodules/PUML/PUML.h:1214:29: error: invalid operands to binary expression ('value_type' (aka 'std::array<const std::vector<int> *, 2>') and 'std::nullptr_t')
 1214 |         assert(allShared[i] != nullptr);
 

[davschneller]

The assert should be fixed now.