Fix min and max vaules in RGBCorrelator if numpy.nan exist in array

[sergey-yaroslavtsev]

To be able to open arrays with "nan".
Nature of problem is that default "min" and "max" could not handle "nan":

>>> c
array([[ 1.        ,  0.        ,         nan],
       [        nan,  1,  3.4       ],
       [-4.        ,  0.        ,  1.        ]])
>>> d
array([[  nan,  0.  , -0.12],
       [ 9.8 ,  1.  ,  3.  ],
       [-4.  ,  0.  ,  1.  ]])
>>> np.ravel(c)         
array([ 1.        ,  0.        ,         nan,         nan,  1,
        3.4       , -4.        ,  0.        ,  1.        ])
>>> np.ravel(d)                                                               
array([  nan,  0.  , -0.12,  9.8 ,  1.  ,  3.  , -4.  ,  0.  ,  1.  ])
>>> min(np.ravel(c))                                                          
np.float64(-4.0)
>>> min(np.ravel(d))    
np.float64(nan)
>>> np.nanmin(np.ravel(d)) 
np.float64(-4.0)