[Bug]: GaussianLogLikelihood returns an error in the case of multiple signals

[NicolaCourtier]

Python Version

3.10.12

Describe the bug

The evaluate and evaluateS1 functions need to be updated for GaussianLogLikelihood to allow with multiple signals. Note that GaussianLogLikelihoodKnownSigma runs without error and produces the expected output.

Steps to reproduce the behaviour

Replace lines 46-47 in the spm_MLE.py example:

problem = pybop.FittingProblem(model, parameters, dataset, signal=["Voltage [V]", "Time [s]"])
likelihood = pybop.GaussianLogLikelihoodKnownSigma(problem, sigma=[0.03, 0.03])

with:

problem = pybop.FittingProblem(model, parameters, dataset, signal=["Voltage [V]", "Time [s]"])
likelihood = pybop.GaussianLogLikelihood(problem)

Running this updated example produces the error message copied below.

Relevant log output

----------------------------------------
Unexpected termination.
Current score: (-inf, -inf)
Current position:
 6.14848282459801054e-01
 4.47667093256804038e-01
----------------------------------------
Traceback (most recent call last):
  File "/home/nicola/GitHub/PyBOP/pybop/costs/base_cost.py", line 88, in evaluate
    return -self._evaluate(x, grad)
  File "/home/nicola/GitHub/PyBOP/pybop/costs/_likelihoods.py", line 148, in _evaluate
    y = self.problem.evaluate(x[: -self.n_outputs])
  File "/home/nicola/GitHub/PyBOP/pybop/_problem.py", line 244, in evaluate
    if np.any(x != self.x) and self._model.matched_parameters:
ValueError: operands could not be broadcast together with shapes (0,) (2,) 

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/nicola/GitHub/PyBOP/examples/scripts/spm_MLE.py", line 54, in <module>
    x, final_cost = optim.run()
  File "/home/nicola/GitHub/PyBOP/pybop/_optimisation.py", line 158, in run
    x, final_cost = self._run_pints()
  File "/home/nicola/GitHub/PyBOP/pybop/_optimisation.py", line 260, in _run_pints
    fs = evaluator.evaluate(xs)
  File "/home/nicola/GitHub/pybop-env/lib/python3.10/site-packages/pints/_evaluation.py", line 119, in evaluate
    return self._evaluate(positions)
  File "/home/nicola/GitHub/pybop-env/lib/python3.10/site-packages/pints/_evaluation.py", line 481, in _evaluate
    scores[k] = self._function(x, *self._args)
  File "/home/nicola/GitHub/PyBOP/pybop/costs/base_cost.py", line 60, in __call__
    return self.evaluate(x, grad)
  File "/home/nicola/GitHub/PyBOP/pybop/costs/base_cost.py", line 94, in evaluate
    raise ValueError(f"Error in cost calculation: {e}")
ValueError: Error in cost calculation: operands could not be broadcast together with shapes (0,) (2,)