Unnecessary warning trigger with linearSimulation

[thast]

Simply running the tutorial plot_inv_1_inversion_lsq.py, I obtain a warning that does not seem to be necessary.

The line sim = simulation.LinearSimulation(mesh, G=G, model_map=model_map)

triggers the warning:

where G seems to be by all mean a valid property... it seems to be more an issue on the order on which piece of code are hit.