Release 1.2.0

[remislp]

Seven merged pull requests have been waiting since 1.1.0, and one of them is now
blocking work elsewhere.

HJCFIT's fitting layer (DCPROGS/HJCFIT#194) is duck-typed on a mechanism and
needs Rate.is_free, which exists here on master and not in the published
1.1.0. Its [fitting] extra pins scalcs>=1.1.0, which is too loose — the
integration tests fail on the released wheel with 'Rate' object has no attribute 'is_free'.

A minor rather than a patch release, because it corrects computed values as
well as adding API.

#17	root bracketing for the asymptotic pdf — counts not recomputed when a bound moved, one-shot widening, the wrong state count, and det[W(s)] unusable at large |s| where it reaches 1e53 and changes sign hundreds of times in noise. Roots now refined by bisecting the eigenvalue count where the determinant cannot be trusted, and validated before being returned
#18	scsim re-exports burst segmentation from dcio — one implementation, not two
#20	the dARSdS sign. exact_mean_open_shut_time was too large by exactly twice the dead time, the conditional means by once. This changes numbers a caller will have seen
#21	impose_resolution checked against RESHJC2.FOR interval for interval
#22	the simulator checked against Q-matrix theory for multi-state mechanisms, not only the two-state case where a reciprocal rate is the answer
#23	qmatlib.H inverted sI − Q_FF directly, which fails exactly where the root search walks. The singularity is removable; only the factored form has a pole
#24	GPL-2.0-or-later, without which HJCFIT (GPL-3.0-or-later) could not depend on this at all

Verified against the built wheel

• reports 1.2.0, both version files bumped — the versions-agree gate from Release 1.1.0 #16
  is what makes that hard to get wrong;
• declares License-Expression: GPL-2.0-or-later;
• carries its three data files (icon, About-box animation, sample mechanism);
• Rate.is_free is present when installed from it into a clean environment,
  which is the thing HJCFIT#194 needs.

617 passing, 1 skipped.

Merge and I will tag v1.2.0.

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