Proposal: changes to the behavior or `Tolerance`/`TOL`

[chenkasirer]

Feature Request

As a developer/user of compas.tolerance, I want changing the global tolerance (TOL) to be explicit and unsurprising so that I don’t accidentally modify global state by calling Tolerance(...).

Details

Is your feature request related to a problem? Please describe.
Right now Tolerance behaves like a singleton: Tolerance() returns TOL, and calling Tolerance(absolute=...) appears to create/configure a new instance but actually mutates the global TOL. This is easy to miss and can cause hard-to-track side effects.

Describe the solution you'd like

Make it very clear when global state is modified. For example:

• Tolerance() (and especially Tolerance(absolute=..., relative=...)) should not implicitly update TOL.
• Provide an explicit API for updating the global tolerance, e.g. TOL.update(...) / TOL.set(...), and/or a dedicated helper like set_default_tolerance(...).
• Possibly, make Tolerance() private or raise error after the first instance has been created.

Describe alternatives you've considered

• Perhaps it should be even possible to create standalone instances of Tolerance in which case Tolerance() actually returns a new instance whereas TOL is the global instance.

Additional context
Current behavior example:

from compas.tolerance import TOL, Tolerance

assert TOL is Tolerance()

tol = Tolerance(absolute=0.01, relative=0.001)  # looks like a new instance
assert tol.absolute == TOL.absolute              # but it actually updated global state
assert tol.relative == TOL.relative