Speed up constant * Expr

[Zeroto521]

As the title. This pr is 1.47x faster than the master branch.

• Optimized before: 6.5767 seconds
• Optimized after: 4.4400 seconds

from timeit import timeit

from pyscipopt import Model


m = Model()

n = 1000
x = m.addMatrixVar(n)
e = x.sum()

number = 100_000
cost = timeit(lambda: e * -1.5, number=number)
print(
    f"Cost of multiplying an expression with a number over {number} runs: {cost:.4f} seconds"
)