Make HeaderPacker pickleable.

HeaderPacker requires special pickling support because the
standard library Struct object is not pickleable. Instead we
pickle the format string extracted from the Struct object when
pickling and use it to recreate the Struct object when
unpickling.

segpy/packer.py

@@ -2,6 +2,7 @@ from collections import OrderedDict
 from struct import Struct
 from itertools import zip_longest
 
+from segpy import __version__
 from segpy.datatypes import SEG_Y_TYPE_TO_CTYPE
 from segpy.util import pairwise, intervals_partially_overlap, complementary_intervals
 
@@ -135,6 +136,27 @@ class HeaderPacker:
         self._structure = structure
         self._field_name_allocations = field_name_allocations
 
+    def __getstate__(self):
+
+        state = self.__dict__.copy()
+        state['__version__'] = __version__
+        state['_structure_format'] = self._structure.format
+        del state['_structure']
+        return state
+
+    def __setstate__(self, state):
+        if state['__version__'] != __version__:
+            raise TypeError("Cannot unpickle {} version {} into version {}"
+                            .format(self.__class__.__name__,
+                                    state['__version__'],
+                                    __version__))
+        del state['__version__']
+
+        structure = Struct(state['_structure_format'])
+        state['_structure'] = structure
+        del state['_structure_format']
+        self.__dict__.update(state)
+
     @property
     def header_format_class(self):
         return self._header_format_class