Revert "Enum names are public."

This reverts commit 6003035a7d27afe33f799799beab363b0ac36a79.

This enum was meant to be private, for internal use only.
What the public needs to know is:
- if weight is lower than 0, then it's unknown
- if weight is infinite, then it's obstacle
- else weight is [0,100]

as documented in weight_map::compute_weight [weight_map.hpp:67]