MultiFab: Overload `+=` `*=` `/=`, etc.

[ax3l]

For pyAMReX' MultiFab, we can overload the operators += *= /=, etc. to call the underlying optimized, element-wise AMReX functions.

Similar for [...] and other set-all ellipsis, we should specialize to AMReX' setVal.

@RemiLehe @roelof-groenewald @dpgrote @atmyers et al.: should we always operate on valid and guard cells for those?

[ax3l]

If have the feeling, if we implemented #370 guards via mf.grow() / mf.with_guard_cells() then we would also have a clear answer for the guard/non-guard question here.

mf = MultiFab()  # from somewhere
mf *= 2.  # on valid cells

mf.with_guard_cells()
mf *= 2.0  # on valid and guard cells

[ax3l]

@roelof-groenewald mentioned today that including guard cells by default is sensible for all but depositions.

[ax3l]

We are looking to in-place update the value of each cell, in each component, including guard cells.
For that, we would implement:

• __iadd__(self, v)
• __isub__(self, v)
• __imul__(self, v)
• __itruediv__(self, v)
• __ifloordiv__(self, v) (for //=)
• __imod__(self, v) (for %)
• __ipow__(self, v)

Example:

        .def("__iadd__",
             [](MultiFab & self, amrex::Real v) { /* ... */ },
             py::is_operator(),
             "Adds in-place to the value of each cell, in each component, including guard cells."
        )

We need to implement scalar and other multifabs for v.