Add check or force Boundary lons/lats to be 1D

Code Sample, a minimal, complete, and verifiable piece of code

from pyresample.boundary import Boundary
b = Boundary(my_lons, my_lats)
print(b.contour_poly.area())

Problem description

The above code doesn't fail if the provided lons/lats are 2D (not sure on 3D+), but the class and all functions/utilities underneath it assume 1D arrays. The end results are incorrect because of this.

Expected Output

Either an exception or a .ravel() on the provided lons/lats so the rest of the calculations work fine.

Actual Result, Traceback if applicable

Incorrect area and other operations.

Start at the Python Boundary class used by the example and inspect how its lons and lats are consumed by contour_poly.area(). Decide whether multidimensional inputs should be rejected or flattened, then verify that the area and related operations no longer silently produce incorrect results.