Able to rerun simulations!

* [QSIM_BUILD,SC] Removed unused cache/2.
* [QSIM_BUILD,NEW] Added add_state/3 for adding a state with a given list of statements.
* [QSIM_ENGINE,SC] Use of add_state/3 from QSIM_BUILD.
* [QSIM_ENGINE,SC] Rewrote the generation of the legal state transitions to be easier debuggable.
* [QSIM_ENGINE,FIX] Several fixes allow simulations to be rerun now!
Improvements to state transition generation.

* [QSIM_ENGINE,EXT] Excluded transitions that decrease in the lowest or increase in the highest landmark.
* [QSIM_ENGINE,EXT] Added transitions that move from a steady into a non-steady same-landmark state. Such states should occur BEFORE an interval can be entered.
* [QSIM_READ,NEW] Added predicates for determining whether a value is an outer landmark or interval.
Integrated a QR-specific notation for QSIM exports (@terminal) and behavior graphs (@SVG).

* [QSIM_BG,EXT] Added `mode=qr` option for vertex labels.
* [QSIM_EXPORT,SC] Simplified the API for qr_state/3 (only one variant remains).
* [QSIM_EXPORT,EXT] Added `mode=qr` option for QR state descriptions.
* [QSIM_EXPORT,NEW] Re-added qr_quantity//4 for writing in QR-specific notation.
* [QSIM_BG,FIX] Due to a PGC update of the DCG-based HTML code, HTML entities are now written correctly inside GraphViz HTML-like labels.