Serilization of expressions is path dependent

[shumpohl]

Currently, qupulse.expressions.sympy.ExpressionScalar remembers the argument with which it was constructed.

This was introduced in 385144d with the goal to "make expression naming more consistent" and allow round-trips like

s = 'a    *    b'
e = Expression(s)
self.assertEqual('Expression(a    *    b)', repr(e))

However, it has the downside of making the repr path dependent. Two equal expressions might have a different repr/serialization representation.

I was not able to reconstruct why we need that repr round-trip functionality.

I would like to either document the reasoning for original_expression in the code or drop it. At least for get_serialization_data.