support custom tolerance

[finchcn]

support custom tolerance

[kunzhoudk]

I will be very good if we can specify the tolerance for relational operators. e.g two parallel lines L1=(0,1),L2 =(0+1.9073486328125E-6,1+1.9073486328125E-6) are overlap when lines is 1.9073486328125E-6 apart.

[stolstov]

That would mean to allow creation of a SpatialReference instance with a custom tolerance.