Pyqrack

The environment variable `QRACK_QUNITMULTI_DEVICES` has been added, as basically a duplicate of the system that parses and logically switches `QRACK_QPAGER_DEVICES`, (_without_ `QRACK_QUNITMULTI_HOST_PTR`, for now or always). Unlike the `QPager` equivalent, `QUnitMulti` will override its environment variable device list by its constructor device list. (This might also ultimately make sense for `QPager`, analogously, but that change could be considered semantically "breaking.")

The logical environment variable interpretation of `QRACK_MAX_PAGE_QB` has been debugged in `QStabilizerHybrid`.

A race condition and a logical edge case were identified and fixed in `QPager`. Upon statistical testing, no indication of `QPager` "edge cases" remains, even under `QUnit`.

Semantically, the `QRACK_SEGMENT_GLOBAL_QB` environment variable has been replaced with `QRACK_MAX_PAGE_QB`. (Rather than specifying a common "segment" level across heterogeneous devices, it makes more sense to specify a common qubit count ceiling.)

This is a patch release for `QPager` debugging and optimization. The API has not (semantically) changed, but `QPager` passes sanity checks for default stack inclusion on `test_mirror_circuit` in the C++11 Qrack benchmark suite.

Bugs were detected and corrected in `QPager`, and parallel dispatch of blocking methods to multiple "pages" was added.

As a part of the PyQrack API, environment variable `QRACK_SEGMENT_GLOBAL_QB` usage expectation might not be backwards compatible from v1.1.0, (as it was debugged).

The Qrack documentation previously stated that the sensible range for the `QRACK_QUNIT_SEPARABILITY_THRESHOLD` environment variable was 0 to 0.5; this was not true. Much earlier in development, this value scaled as probability to be in |1> (or |0>) eigenstate, hence the older instructions. Cleaning up the consistent use of the parameter, it actually _now_ represents (1 minus) the length of a state projected on a local qubit Bloch sphere, assumed to have unit radius. (So, the sensible values of the environment variable are 0 to 1, and have been for some time, up to consistent interpretation of the parameter's meaning by Qrack internals.).

Measurement sampling has also been optimized, such that, when all qubits are being measured, `QUnit` does not need to flush _any_ entangling buffers to produce the measurement distribution, such that subsystem footprints do not change in qubit count at all upon sampling.