Fix expressions with duration literals

[anuragm]

Duration literals are missing a type which causes errors when trying to build expressions with them. For example, this code

import oqpy

port = oqpy.PortVar("myport")
frame = oqpy.FrameVar(port, 1e9, name="myframe")
delay_time = oqpy.make_duration(50e-9) # 50 ns
delay_repetition = 10

program = oqpy.Program()
program.delay(delay_repetition * delay_time, frame)

errors out and print

AttributeError                            Traceback (most recent call last)
Cell In[2], line 9
      6 delay_repetition = 10
      8 program = oqpy.Program()
----> 9 program.delay(delay_repetition * delay_time, frame)

File ~/Work/oqpy/oqpy/base.py:96, in OQPyExpression.__rmul__(self, other)
     95 def __rmul__(self, other: AstConvertible) -> OQPyBinaryExpression:
---> 96     return self._to_binary("*", other, self)

File ~/Work/oqpy/oqpy/base.py:61, in OQPyExpression._to_binary(op_name, first, second)
     56 @staticmethod
     57 def _to_binary(
     58     op_name: str, first: AstConvertible, second: AstConvertible
     59 ) -> OQPyBinaryExpression:
     60     """Helper method to produce a binary expression."""
---> 61     return OQPyBinaryExpression(ast.BinaryOperator[op_name], first, second)

File ~/Work/oqpy/oqpy/base.py:237, in OQPyBinaryExpression.__init__(self, op, lhs, rhs)
    235     self.type = lhs.type
    236 elif isinstance(rhs, OQPyExpression):
--> 237     self.type = rhs.type
    238 else:
    239     raise TypeError("Neither lhs nor rhs is an expression?")

AttributeError: 'OQDurationLiteral' object has no attribute 'type'

Adding the proper type fixes the issue. I have also added a few tests.