Nionswift-usim

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- Improve simulated synchronization behavior for future work.
- Fix minor issues with camera length.

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- Change camera device to implement camera 3 (no prepare methods).

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- Updates for nionswift-instrumentation 0.20 and nionswift 0.16.
- Add support for axis descriptors.
- Add support for camera masks.
- Drop support for Python 3.7.

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- Add support for partial synchronized acquisition.
- Fix handling of probe position in sub-scans.
- Add aperture that can be moved and "distorted" (i.e. dipole and quadrupole effect simulation).
- Add functions to 'Instrument' that facilitate adding new inputs to existing controls.
- Allow input weights for controls to be controls in addition to float.
- Add option to attach a python expression as control input (only one expression per control can be set, but it can be arbitrarily complex, as long as it can be evaluated by 'eval').
- Changed meaning of convergence angle to reflect its real meaning (in the simulator it only controls the size of the aperture on the Ronchigram camera, the effect on the scan is not simulated yet).
- Add 'Variable' class to InstrumentDevice. 'Variables' differ from 'Controls' in that they do not have a local value.

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- Minor changes to be compatible with nionswift-instrumentation.
- Improve 'inform' functionality on Ronchigram controls.

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- Fix some simulated aberration calculations.
- Add option for flake sample (same as previous version) or amorphous sample.
- Allow adding new controls to existing instrument instance.
- Add support for 2D controls and AxisManager.