Included PolarGrid class. right now it assumes no ghost cells

[ssagynbayeva]

[zingale]

I think that this is the geometry you want:

[zingale]

you should only need to add 3 functions to Grid2d:

area_x() area_y() volume()

it might also be good to add an assertion on the angle (y)

the ghost cells should be handled just fine by what is already in the class