Avoid loading OpenCV when not needed

[AdeelH]

from mask_to_polygons.processing import buildings

causes cv2 to be imported, and for that to succeed, you need a number of other packages/libraries installed. Example errors:

from .cv2 import * ImportError: libgthread-2.0.so.0: cannot open shared object file: No such file or directory
from .cv2 import * ImportError: libGL-2.0.so.0: cannot open shared object file: No such file or directory
from .cv2 import * ImportError: libSM-2.0.so.0: cannot open shared object file: No such file or directory

This is too much hassle if one is only interested in using polygons and not buildings.

This PR moves the above import statement, so that it is only run if the user wants to use buildings. It does the same for polygons for consistency.