Create square, cube, disk, and sphere functions in tools since scikit-image is deprecating them

[mkaguer]

Let's use our own porespy structuring elements to avoid deprecation issue in the future and to avoid using long winded name.

Everywhere we do from skimage.morphology import square, cube needs to be replaced with porespy's

[jgostick]

Issue #1057 is also relevant here. Those functions could be added to the same place.