Numpy boolean indexing not working on Field object

[skuschel]

just noticed that the boolean indexing on Field objects is not working. See https://numpy.org/doc/stable/user/basics.indexing.html
This is generally undesired, because it breaks the regular axes, however capping values beyond a threshold would be good thing, like

import postpic as pp
import numpy as np

f = pp.Field(np.random.random((100, 101)))
f[f<0.5] = 0

currently terminate with an error:

File [~/.venv/py311/lib/python3.11/site-packages/postpic/datahandling.py:2312](http://localhost:8888/home/stephan/.venv/py311/lib/python3.11/site-packages/postpic/datahandling.py#line=2311), in Field._normalize_slices(self, key)
   2310     key = (key,)
   2311 if len(key) != self.dimensions:
-> 2312     raise IndexError("{}D Field requires a {}-tuple of slices as index"
   2313                      "".format(self.dimensions, self.dimensions))
   2315 return tuple(ax._normalize_slice(sl) for ax, sl in zip(self.axes, key))

IndexError: 2D Field requires a 2-tuple of slices as index

postpic version 0.5+58.g315634f