REFACTOR Representing hyperparameters theta as 1D ndarrays instead of 2D 1xn arrays
FIX Setting optimal_rlf_value correctly in GPs and actually maximizing likelihood over several random starts.

The sign of the optimal_rlf_value was set wrongly in 
_arg_max_reduced_likelihood_function() since it was set to the negative
of the return of reduced_likelihood_function(). This error was probably
caused by confusing reduced_likelihood_function(theta) 
with minus_reduced_likelihood_function(log10t). It resulted, however,
in _arg_max_reduced_likelihood_function() returning the worst local
maxima instead of the best one when performing several random_starts.
FIX Adapted Welch optimizer to 1D shape of theta introduced in 4f4726