huge update:

introduced meta tasks which are used by `_map()`;
introduced task groups;
ending with `gather_and_close()` now;
pool unittests rewritten accordingly;
two new helper classes

src/asyncio_taskpool/exceptions.py

@@ -23,6 +23,10 @@ class PoolException(Exception):
     pass
 
 
+class PoolIsClosed(PoolException):
+    pass
+
+
 class PoolIsLocked(PoolException):
     pass
 
@@ -43,6 +47,10 @@ class InvalidTaskID(PoolException):
     pass
 
 
+class InvalidGroupName(PoolException):
+    pass
+
+
 class PoolStillUnlocked(PoolException):
     pass