code refactor

related to #8 (closed):

• I kept changes per commit small, so the number of commits got kind of numerous...

src/icartt/dataset.py

@@ -71,7 +71,7 @@ class DataStore1001:
             if key in self.varnames:
                 newline[key] = vmiss_to_npnan(kwargs[key], self.missvals[key])
 
-        if type(self.data) is type(None): # TODO: None is the sole instance of None ;-) => can use "if self.data is None"
+        if self.data is None:
             self.data = newline
             self.data = self.data.reshape(1)  # don't even ask
         else: