Remove add and sub from Rdate

They were not symmetrical, better to remove them.

CHANGELOG

@@ -1,4 +1,5 @@
 3.0
+- Dates can no longer be added or subtracted
 - Types are now inferred by column, no longer by cell
 - Relations now use frozenset internally and are immutable
 - Refactored parser to use better typing

relational/rtypes.py

@@ -88,10 +88,6 @@ class Rdate:
     def __str__(self):
         return self.intdate.__str__()
 
-    def __add__(self, days):
-        res = self.intdate + datetime.timedelta(days)
-        return Rdate(res.__str__())
-
     def __eq__(self, other):
         if not isinstance(other, Rdate):
             return False
@@ -109,9 +105,6 @@ class Rdate:
     def __lt__(self, other):
         return self.intdate < other.intdate
 
-    def __sub__(self, other):
-        return (self.intdate - other.intdate).days
-
 
 def is_valid_relation_name(name: str) -> bool:
     '''Checks if a name is valid for a relation.