Changed the property that holds the minimum age required for Students to participate in a GHOP based program. Added a new property student_min_age_as_of to hold the DateTime from when the minimum age should be reached. Changed student_min_age property to now store the student minimum age as an integer. This all feel more user friendly then letting the program admin calculate the required DateTime. Storing the two properties is then still needed in this case since information might be lost otherwise when the program admin brings up the edit view. Reviewed by: Lennard de Rijk

# Until model subclassing is a possibility, a mixin class is used to add
# the necessary functions that may be contributed for geographic objects.
class GeoMixin:
    """
    The Geographic Mixin class provides routines for geographic objects,
    however, it is no longer necessary, since all of its previous functions 
    may now be accessed via the GeometryProxy.  This mixin is only provided
    for backwards-compatibility purposes, and will be eventually removed
    (unless the need arises again).
    """
    pass