API: always recompute the volume in polytope.polytope.volume

because this is implicit. Explicit is better than implicit [PEP 20][pep20].
Doing so surprises the user, and is a silent error in case the `Polytope`
or `Region` has been modified by the user since the last time its
volume was computed. Errors should never pass silently [PEP 20][pep20].

The attribute `volume` of the classes `Polytope` and `Region` is
a property that is cached in the attribute `_volume`. Therefore,
users know and expect that accessing this attribute will return
a cached value.

However, what is nonobvious to users, and would be unexpected,
is that also when calling the function `polytope.polytope.volume`,
still the polytope's cached value is returned, if present.
A user would expect that calling the function `volume` would
compute the volume again, because that is an explicit call to the
volume computation algorithm.

More fundamentally, without this change, it becomes impossible
for a user to recompute a polytope's volume without resetting
the attribute `_volume` to `None` in user code. This would require
that users use internal package details for updating the volume.

Do log a debug-level message for tracking the computation.

The `try..except` was a bug, because it masked an `Exception`.
There is no reason why `polyreg` would not have `_volume` as attribute
(both `Polytope` and `Region` have such an attribute).
Even if `polyreg` was required to have an attribute named `_volume`,
still catching `Exception` is too broad. Instead, `AttributeError`
would be caught.

[pep20]: https://www.python.org/dev/peps/pep-0020/

polytope/polytope.py

@@ -1477,11 +1477,8 @@ def volume(polyreg, nsamples=None):
     """
     if not is_fulldim(polyreg):
         return 0.0
-    try:
-        if polyreg._volume is not None:
-            return polyreg._volume
-    except Exception:
-        logger.debug('computing volume...')
+    if polyreg._volume is not None:
+        logger.debug('recomputing polytope volume...')
     # `Region` ?
     if isinstance(polyreg, Region):
         tot_vol = 0.