Add Position Data to ADIOS-Formatted Molecular Dataset

[zachfox]

This pull request adds position data to the molecular dataset stored in ADIOS format. The new implementation includes:

•   Position data integration: Position (x, y, z) coordinates for each atom in the molecular dataset are now included.
•   Backward compatibility: Ensured that datasets without position data can still be read without errors.

I checked that the positions were added to the ADIOS files with bpls -a data.bp:

 int64_t  testset/ndata                        attr
  float    testset/pos                          {7198, 3}
  int64_t  testset/pos/variable_count           {146}
  int64_t  testset/pos/variable_dim             attr
  int64_t  testset/pos/variable_offset          {146}
  float    testset/x                            {7198, 6}
  int64_t  testset/x/variable_count             {146}
  int64_t  testset/x/variable_dim               attr
  int64_t  testset/x/variable_offset            {146}
  double   testset/y                            {292}