Initial DeriveUnit implementation

This contains the initial DeriveUnit implementation, including signature
validation, this initial implentation only supports ValueDescriptors in
the signature alongside the SignalDescriptor and returns a single
ValueDescriptor

The worker thread still needs to be defined and started but the offline
processing has already been validate to work as a first prototype

src/dynalab_core/errors.py

@@ -18,3 +18,16 @@ class DLPakError(Exception):
 
 class DLPakNoDataError(DLPakError):
     """DLPak no data error."""
+
+
+# DeriveUnit error declarations
+class DeriveUnitError(Exception):
+    """DeriveUnit error."""
+
+
+class DeriveUnitInvalidSignatureError(DeriveUnitError):
+    """DeriveUnit invalid processing function signature error."""
+
+
+class DeriveUnitArgsMismatchError(DeriveUnitError):
+    """DeriveUnit incorrect arguments provided error."""