This function aims to find the min distance (ie closest point) to any other x (numeric value), ie intra 'x' and
returns matrix with 'index','value','dif','ppm','ncur','nbest','best'.
At equal distance to lower & upper neighbour point, the upper (following) point is chosen (as single best).
In case of multiple ex-aequo distance returns 1st of multiple, may be different at various repeats.