calc_md with pressure 0 corrected

[samwaseda]

Pointed out here

[samwaseda]

Don't merge it yet I'm completing the unit tests

[jan-janssen]

======================================================================
4024ERROR: test_dump_parser_water (lammps.test_base.TestLammps)
4025----------------------------------------------------------------------
4026Traceback (most recent call last):
4027  File "/home/travis/build/pyiron/pyiron/tests/lammps/test_base.py", line 286, in test_dump_parser_water
4028    pressure=0,
4029  File "/home/travis/build/pyiron/pyiron/pyiron/lammps/interactive.py", line 291, in calc_md
4030    delta_press=delta_press,
4031  File "/home/travis/build/pyiron/pyiron/pyiron/lammps/base.py", line 662, in calc_md
4032    rotation_matrix=self._prism.R
4033  File "/home/travis/build/pyiron/pyiron/pyiron/lammps/control.py", line 504, in calc_md
4034    pressure = self.pressure_to_lammps(pressure, rotation_matrix)
4035  File "/home/travis/build/pyiron/pyiron/pyiron/lammps/control.py", line 233, in pressure_to_lammps
4036    raise ValueError("Cells which are not orthorhombic or rectangular are incompatible with Lammps "
4037ValueError: Cells which are not orthorhombic or rectangular are incompatible with Lammps constant pressure calculations unless the entire pressure tensor is defined.The reason is that Lammps demands such cells be represented with an upper-triangular unit cell, thus a rotation between Lammps and pyiron coordinateframes is required; it is not possible to rotate the pressure tensor if any of its components is None.
4038
4039----------------------------------------------------------------------

[jan-janssen]

Looks good to me and solves my issue