BUG Add tol to _make_unique to avoid inf values in IsotonicRegression - #18639

Reference Issues/PRs

Fixes #10903
Closes #10981 and #17758

What does this implement/fix? Explain your changes.

Add a tol when checking equality in _make_unique, such that values are deemed unique only if their difference is greater or equal than 1e-15 i.e.:

if x - current_x >= 1e-15:

this agrees with Python:

>>> 1 == 1+1e-15
False
>>> 1 == 1+1e-16
True

Note that this is not the case for 0:

>>> 0 == 1e-15
False
>>> 0 == 1e-323
False
>>> 0 == 1e-324
True

see more here: #17758 (comment)

Any other comments?

Also removed current_count as it does not seem to be used anymore (as discussed with @ogrisel)
cc @ogrisel