Lightworks

This release added a set of standard components for implementing photonic qubit gates and expanded on the documentation.

Full change log:
- Added single and two qubit gate circuits which can be used to quickly implement required qubit processing.
- Improved docs, including sections of backend usage, extra Simulator examples and some information on new qubit components. Also corrected some grammatical issues.
- Removed some redundant functions and fidelity calculation as this was not well defined.
- Added slos to States and AnnotatedStates to reduce memory usage, also removed pre-calculation of mode number.
- Added method in state to perform validation on the values, checking that they are all integers and are not negative.
- Adjusted the indexing method used in SimualtionResult, now values are accessed with [input, output] instead of [input:output].

This release of Lightworks reworks the backend to provide a framework for a future expansion of backend options. A basic SLOS implementation has also been added to the Sampler for initial testing of the backend functionality.

There was also a number of additional updates in this release:
- Improved performance of some components.
- Expanded on unit tests, including using parametrization in pytest to quickly check a range of values.
- Updated docs, detailing the Unitary object and adding a demo notebook for the heralded CNOT.

Initial release of Lightworks