Show non-significant (i.e. net-stable) points in WMS

[robbibt]

This would allow users to click on a non-eroding or non-prograding beach and still get an informative graph back showing temporal change.

This would be particularly useful for wave-dominated beaches without persistent temporal trends, or where coastline movement is driven by fluctuating climatic drivers.

A mockup of how this would function (see also #39, #30, #29):

if wms_t_sig:

    if wms_t_grew:
        beach grew by wms_t_abs (+- wms_t_conf) 
    
    if wms_t_retr:
        beach eroded by wms_t_abs (+- wms_t_conf) 
    
    if wms_t_less:
        this rate was smaller than X warning        
    
if wms_t_nsig:

    beach was net stable since 1988 (e.g. no retreating or growing trend)