Add test for BigInteger NRoot
Add test for BigInteger NRoot

This also required changing the testIntegralNRoot helper method to not
require a Numeric instance, since we do not (yet) have a Numeric[BigInteger]

guersam/spire,non/spire,adampingel/spire,JackSullivan/spire,codeaudit/spire,siddhartha-gadgil/spire,guersam/spire,kevinmeredith/spire,JackSullivan/spire,siddhartha-gadgil/spire,codeaudit/spire,kevinmeredith/spire,tixxit/spire,rklaehn/spire,rklaehn/spire,tixxit/spire,adampingel/spire,non/spire