Add UnfilledModalError, records.verify_modal(), and .get_*() methods.

The .get_*() methods are used to verify that we aren't reading from a
pattern with un-filled modals.

The GeometryMixin class was also added here and provides some additional
convenience methods: get_xy() to get an (x,y) tuple and
get_layer_tuple() to get a (layer, datatype) tuple.

fatamorgana/__init__.py

@@ -29,6 +29,7 @@ import pathlib
 from .main import OasisLayout, Cell, XName
 from .basic import NString, AString, Validation, OffsetTable, OffsetEntry, \
         EOFError, SignedError, InvalidDataError, InvalidRecordError, \
+        UnfilledModalError, \
         ReuseRepetition, GridRepetition, ArbitraryRepetition