Provide implementation of right-hand side binary operations

[Markus-Goetz]

At the moment it is possible to do the following for example:

a = ht.zeros((5, 3))
a + 1

However, it is impossible to do:

a = ht.zeros((5, 3))
1 + a

HeAT currently does not provide implementation for so-called right-hand side operations, e.g. __radd__ etc.