Separate data structs for NPt Parameters and instantaneous parameters

[RudolfWeeber]

In Preparation for NPt rework.

Both, the parameters (.e.g. Piston mass, external pressrure) and cached pressure values from last force calculation (instantaneous pressure, p_inst) are mixed up in a singel struct NPTIsoParameters in src/core/npt.hpp

• Step 1: Split NPTIsoParameters into NPTIsoParameters and InstantaneousPressure, so that the firest one really only contains parametes of the algorithm and the second one contains the pressure that is calculated on the fly during force calculaiton.
  Note: The easiest way to refactor is to temporarily usie two globals, the current nptiso and a new npt_inst_pressure.
• step 2: get rid of the two globals by making them leafs of the System class