Test the evaluation of multidimensional gradients
Test the evaluation of multidimensional gradients

Generalize the tests in testGradient.cxx to receive model functions
of one or two dimensions. There are now 18 tests, with all the possible
combinations of:

 * Type of gradient: Chi2, LogL or PoissonLogL
 * Number of dimensions: 1 or 2
 * Data type: scalar or vectorial
 * Execution policy: serial or multithread

excluding the scalar serial case, which is computed as the base case
to compare the output of the other ones.

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