Fixed problem where RFs would sometimes not be displayed

Under certain conditions the AttrDict holding the RFs would not
behave correctly, preventing attribute access from working. This
commit now uses the dictionary get method, which still works under
these conditions. We should still investigate, why the AttrDict
misbehaved under these circumstances (measuring RFs after loading
a snapshot).
Added projection grid measurement command and class method

Projections now have method to generate a grid of CFs, which sample
the underlying CF grid at some defined resolution. This method is
aliased by a measurement command.