Remove hidden open warning from vlsv_reader_parallel.h

[iljah]

GCC 16.1.1 gives:

In file included from vlsv_reader_parallel.h:24,
                 from vlsv_reader_parallel.cpp:25:
vlsv_reader.h:46:20: warning: ”virtual bool vlsv::Reader::open(const std::string&)” was hidden [-Woverloaded-virtual=]
   46 |       virtual bool open(const std::string& fname);
      |                    ^~~~
vlsv_reader_parallel.h:45:12: note:   by ”bool vlsv::ParallelReader::open(const std::string&, MPI_Comm, const int&, MPI_Info)”
   45 |       bool open(const std::string& fname,MPI_Comm comm,const int& masterRank,MPI_Info mpiInfo=MPI_INFO_NULL);
      |            ^~~~

[petschge]

I think this deserves a comment in to code why it's there (and how it works). Just shutting up warnings so they are gone is only half the goal.

[iljah]

What happens is mentioned in https://en.cppreference.com/cpp/language/virtual but https://stackoverflow.com/a/4146525 explains it better, I'll let someone else decide how to handle this.

[iljah]

added comments to both instances