ConfigurationPoints: Added check that default values are POD's

Default config values will now need to be a Plain Old Data type.
This commit includes some ultility functions to help with this check

wlauto/core/configuration/configuration.py

@@ -24,6 +24,7 @@ from wlauto.utils.types import (identifier, integer, boolean,
                                 list_of_strings, toggle_set,
                                 obj_dict)
 from wlauto.core.configuration.tree import SectionNode
+from wlauto.utils.serializer import is_pod
 
 # Mapping for kind conversion; see docs for convert_types below
 KIND_MAP = {
@@ -228,6 +229,9 @@ class ConfigurationPoint(object):
             raise ValueError('Kind must be callable.')
         self.kind = kind
         self.mandatory = mandatory
+        if not is_pod(default):
+            msg = "The default for '{}' must be a Plain Old Data type, but it is of type '{}' instead."
+            raise TypeError(msg.format(self.name, type(default)))
         self.default = default
         self.override = override
         self.allowed_values = allowed_values
@@ -798,7 +802,7 @@ class RunConfiguration(Configuration):
                            the connected device. Obviously, this must match your setup.
                            '''),
         ConfigurationPoint('retry_on_status', kind=status_list,
-                           default=status_list(['FAILED', 'PARTIAL']),
+                           default=['FAILED', 'PARTIAL'],
                            allowed_values=ITERATION_STATUS,
                            description='''
                            This is list of statuses on which a job will be cosidered to have failed and

wlauto/utils/serializer.py

@@ -7,7 +7,7 @@ structures and Python class instances).
 
 The modifications to standard serilization procedures are:
 
-    - mappings are deserialized as ``OrderedDict``\ 's are than standard
+    - mappings are deserialized as ``OrderedDict``\ 's rather than standard
       Python ``dict``\ 's. This allows for cleaner syntax in certain parts
       of WA configuration (e.g. values to be written to files can be specified
       as a dict, and they will be written in the order specified in the config).
@@ -61,15 +61,27 @@ __all__ = [
     'read_pod',
     'dump',
     'load',
+    'is_pod',
+    'POD_TYPES',
 ]
 
+POD_TYPES = [
+    list,
+    tuple,
+    dict,
+    set,
+    basestring,
+    int,
+    float,
+    bool,
+    datetime,
+    regex_type
+]
 
 class WAJSONEncoder(_json.JSONEncoder):
 
     def default(self, obj):  # pylint: disable=method-hidden
-        if hasattr(obj, 'to_pod'):
-            return obj.to_pod()
-        elif isinstance(obj, regex_type):
+        if isinstance(obj, regex_type):
             return 'REGEX:{}:{}'.format(obj.flags, obj.pattern)
         elif isinstance(obj, datetime):
             return 'DATET:{}'.format(obj.isoformat())
@@ -241,3 +253,7 @@ def _read_pod(fh, fmt=None):
         return python.load(fh)
     else:
         raise ValueError('Unknown format "{}": {}'.format(fmt, getattr(fh, 'name', '<none>')))
+
+
+def is_pod(obj):
+    return type(obj) in POD_TYPES