corrections in quickstart for mpirun (#132)

**Solves the following issue(s):**

Closes #130  and #131

Author: spossann

doc/sections/quickstart.rst

@@ -69,7 +69,8 @@ Under ``simdata.f`` you find binning data, in this case a 1d binning plot in the
 
 Parallel simulations can invoked from the same launch file for instance by::
 
-    mpirun -n 4 struphy_test.py
+    pip install -U mpi4py
+    mpirun -n 4 python struphy_test.py
 
 If you want to learn more please check the :ref:`userguide`.
 

src/struphy/models/base.py

@@ -16,7 +16,12 @@
 from struphy.feec.basis_projection_ops import BasisProjectionOperators
 from struphy.feec.mass import WeightedMassOperators
 from struphy.feec.psydac_derham import SplineFunction
-from struphy.fields_background.base import FluidEquilibrium, FluidEquilibriumWithB, MHDequilibrium
+from struphy.fields_background.base import (
+    FluidEquilibrium,
+    FluidEquilibriumWithB,
+    MHDequilibrium,
+    NumericalMHDequilibrium,
+)
 from struphy.fields_background.equils import HomogenSlab
 from struphy.fields_background.projected_equils import (
     ProjectedFluidEquilibrium,
@@ -95,7 +100,7 @@ def setup_domain_and_equil(self, domain: Domain, equil: FluidEquilibrium):
         """If a numerical equilibirum is used, the domain is taken from this equilibirum."""
         if equil is not None:
             self._equil = equil
-            if "Numerical" in self.equil.__class__.__name__:
+            if isinstance(self.equil, NumericalMHDequilibrium):
                 self._domain = self.equil.domain
             else:
                 self._domain = domain