Use more C++ style memory management for plot_info: Use std::vector
for array data. Return the plot_info instead of filling an output
parameter. Add a constructor/destructor pair so that the caller
isn't bothered with memory management.
The bulk of the commit is replacement of pointers with references,
which is kind of gratuitous. But I started and then went on...
Default initializiation of gas_pressures made it necessary to convert
gas.c to c++, though with minimal changes to the code.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>