CircuitResult compatible with simulation

[stavros11]

Linked to qiboteam/qibo#618. Implements the following backend methods:

• circuit_result_representation: Defines how a circuit result is printed. Currently just prints the readout dictionary as a string.
• circuit_result_probabilities: Calculates output probabilities currently based on the naive voltage normalization from the main branch.

The following script now works in slurm:

import qibo
from qibo import gates, models

# Create circuit
c = models.Circuit(1)
c.add(gates.Y(0))
c.add(gates.M(0))

# Execute with numpy
qibo.set_backend("numpy")
for _ in range(5):
    result = c(nshots=1024)
    print(result)
    print(result.probabilities())

# Execute on hardware
from qibolab.paths import qibolab_folder
runcard = qibolab_folder / 'runcards' / 'qw5q.yml'
qibo.set_backend("qibolab", platform="tiiq", runcard=runcard)

for _ in range(5):
    result = c(nshots=1024)
    print(result)
    print(result.probabilities())

so there is symmetry between qibolab and simulation backends.

Similar implementations should be done for result.samples() and result.frequencies().