Fix MultiFab getitem for single imaginary index (#477)

This fixes the indexing of MultiFabs when a single imaginary index is
given, for example `Ex[1j]`.

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: dpgrote

src/amrex/extensions/MultiFab.py

@@ -312,7 +312,9 @@ def _process_index(self, index):
 
     if index == Ellipsis:
         index = []  # This will be filled in below to cover the valid cells
-    elif isinstance(index, slice) or isinstance(index, int):
+    elif (
+        isinstance(index, slice) or isinstance(index, int) or isinstance(index, complex)
+    ):
         # If only one slice or integer passed in, it was not wrapped in a tuple
         index = [index]
     elif isinstance(index, tuple):