Add possibility to multiply SymbolicHamiltonian by scalar or add possibility to __add__ together with scalar rescaling

[marekgluza]

I noticed that there is a nice feature of adding symbolic Hamiltonians but for many applications adding after rescaling by scalars is also useful.

Example

import qibo
from qibo.symbols import *
from qibo.hamiltonians import SymbolicHamiltonian

# works
h1 = SymbolicHamiltonian( Z(0), nqubits = 2)
h2 = SymbolicHamiltonian( Z(1), nqubits = 2)

h12 = h1 + h2
print(h12.form)

# works

h12 = SymbolicHamiltonian( 0.1 * Z(0) + 0.2 * Z(1), nqubits = 2)
print(h12.form)

# fails

h12 = 0.1 * h1 + 0.2 * h2

The error message is inaccurate because both Hamiltonians had the same nqubits

[Qibo 0.2.12|ERROR|2024-11-07 10:40:47]: Only hamiltonians with the same number of qubits can be added.

[renatomello]

@marekgluza the fix is in #1517