Fix non-virtual destructor warning in ProgressPrinter base class

[SpatLyu]

This PR adds a virtual destructor to the ProgressPrinter abstract base class to resolve a Clang warning observed on macOS:

clang++ -arch arm64 -std=gnu++17 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG  -I'/Volumes/Builds/templates/cran-build/big-sur-arm64/Rlib/4.6/Rcpp/include' -I'/Volumes/Builds/templates/cran-build/big-sur-arm64/Rlib/4.6/RcppThread/include' -I'/Volumes/Builds/templates/cran-build/big-sur-arm64/Rlib/4.6/RcppArmadillo/include' -I/opt/R/arm64/include   -DARMA_USE_CURRENT -fPIC  -falign-functions=64 -Wall -g -O2   -c SCPCM4Grid.cpp -o SCPCM4Grid.o
In file included from PatternCausality.cpp:1:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/vector:321:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/__format/formatter_bool.h:17:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/__format/concepts.h:17:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/__format/format_parse_context.h:16:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/string_view:1059:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/algorithm:1778:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/__algorithm/inplace_merge.h:28:
/Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/__memory/unique_ptr.h:68:5: warning: delete called on non-final 'RcppThread::ProgressBar' that has virtual functions but non-virtual destructor [-Wdelete-non-abstract-non-virtual-dtor]
   68 |     delete __ptr;
      |     ^
/Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/__memory/unique_ptr.h:300:7: note: in instantiation of member function 'std::default_delete::operator()' requested here
  300 |       __ptr_.second()(__tmp);
      |       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include/c++/v1/__memory/unique_ptr.h:266:75: note: in instantiation of member function 'std::unique_ptr::reset' requested here
  266 |   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_SINCE_CXX23 ~unique_ptr() { reset(); }
      |                                                                           ^
PatternCausality.cpp:754:44: note: in instantiation of member function 'std::unique_ptr::~unique_ptr' requested here
  754 |   std::unique_ptr bar;
      |         

Since ProgressPrinter declares a pure virtual function (printProgress()), it is used polymorphically. Deleting derived objects through a base-class pointer (e.g., via std::unique_ptr<ProgressPrinter>) without a virtual destructor results in undefined behavior and triggers this warning.

Adding

virtual ~ProgressPrinter() = default;

ensures correct destruction of derived instances. Because ProgressPrinter is already a polymorphic type, this change does not introduce any additional runtime overhead.

[tnagler]

Thanks!