Handle singular matrices

[JCGoran]

In some cases, such as parameter degeneracy, the Fisher matrix is singular and cannot be inverted; this has an impact on certain methods:

• FisherMatrix.inverse
• FisherMatrix.marginalize_over
• FisherMatrix.correlation_matrix
• FisherMatrix.constraints
• FisherBaseFigure.plot
• plot_curve_1d
• plot_curve_2d

The code should catch these errors (numpy.LinAlgError), and handle this somehow. Some suggestions (a combination of these would also work):

• show a UserWarning
• when calling any plotting methods, plot parallel lines; this is a bit challenging as autoscale_view would make anything else drawn on the axis look tiny, but this can be fixed somehow (see this and this SO questions).
• something else?

[JCGoran]

Maybe one can use a Moore-Penrose inverse instead of a regular inverse?