make setMinMax public to allow for digest creation by library users

[johnistan]

I have a use case where I am serializing digests from non-jvm envs and want to be able to create jvm digests from pre-merged list of centroids. the add(double x, int w); methods allow for adding centroids but I am unable to set the min/max from out the package.

PR makes the setMinMax to allow external users to manually set these member variables.