Make the Section types work with the coordinates of the arrays from xarray

[amoodie]

for example, when creating a StrikeSection, y=10 should be at the actual coordinate y==10, not the 10th cell in the y direction. This will be a breaking change, but I feel it is the correct way to handle this as input.

One option would be to implement another option y_cell=10 that is mutually exclusive with y=, and maintains the functionality of the current api.