Allow wt.artists.create_figure() hspace and wspace arguments to accept lists/arrays

[DLafayetteII]

Optimal spacing between subplots may differ within one figure depending on axes labels and tick label widths. A common example of this difference in preferred spacing is that the colorbar may not require the same spacing from the adjacent subplots as the subplots need from each other:

Current implementation only allows for one consistent height (or width) spacing between all subplots of a figure. Enabling wt.artists.create_figure() to accept a 1D list of values for hspace and wspace would enhance figure generation.

[DLafayetteII]

For anyone interested in circumventing this limitation in the mean time, @ddkohler suggested making spacing minimal, say wspace = 0, and then using thin subplot columns, i.e. col = [1, 0.05, 1, cbar] where you do want spacing. There may be games to play with the aspect ratio argument as well (haven't thought that far).

[DLafayetteII]

Looks like we are currently dependent on matplotlib.gridspec.Gridspec, which does not accept a list nor array of wspace/hspace.