Fixed so YAML plugin checks are seen as valid by main compliance-checker

[agstephens]

A couple of lines changed, to make our Base Check class match the signature of the main Check Base class in the compliance checker:

diff --git a/checklib/register/callable_check_base.py b/checklib/register/callable_check_base.py
index 41df325..5a2376f 100644
--- a/checklib/register/callable_check_base.py
+++ b/checklib/register/callable_check_base.py
@@ -1,3 +1,7 @@
+
+from netCDF4 import Dataset
+
+from compliance_checker import MemoizedDataset
 from compliance_checker.base import BaseCheck, Dataset, Result
 from checklib.code.errors import FileError, ParameterError

@@ -13,7 +17,7 @@ class CallableCheckBase(object):
     required_args = []
     message_templates = []
     level = BaseCheck.HIGH
-    supported_ds = [Dataset]
+    supported_ds = {Dataset, MemoizedDataset}