Calculate probability and export to gis raster/grid
        format. 
        
        export_probability(prob_out,
                           trajectory_files = [myfiles1.nc, myfiles2.nc],
                           bbox = [-75, 23, -60, 45],
                           nx = 1000, ny = 1000,
                           method = 'overall')

def export_probability(outputname, **kwargs):
    """
        Calculate probability and export to gis raster/grid
        format. 
        
        export_probability(prob_out,
                           trajectory_files = [myfiles1.nc, myfiles2.nc],
                           bbox = [-75, 23, -60, 45],
                           nx = 1000, ny = 1000,
                           method = 'overall')  
    """
    bbox = kwargs.get('bbox', None)
    nx, ny = kwargs.get('nx', None), kwargs.get('ny', None)
    if bbox == None:
        raise ValueError('Must supply bbox keyword argument.')
    if nx == None or ny == None:
        raise ValueError('Must supply nx and ny keyword arguments.')
    prob = compute_probability(**kwargs)
    export_grid(outputname, prob, bbox, nx, ny)