Pyproprop

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Changed
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- Move CI to GitHub Actions. Remove use of Appveyor and TravisCI.

Fixed
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- Fix bug which accidentally added the test directory as an additional package in the distribution causing conflicts with builds on conda-forge.

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Fixed
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- Fix bug which caused `iterable_allowed` `processed_property` objects to be cast incorrectly.
- Add tests for `iterable_allowed` `processed_property` objects to new test module `tests/unit/processed_property/test_iterable.py`.

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Added
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- Add `is_optimisable` and `is_read_only` boolean attributes to `processed_property` class attributes.
- Add tests to `tests/unit/processed_property/test_optimisable.py` testing optimisable processed properties.

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Added
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- New test module `tests/unit/processed_property/test_default.py` for processed properties with default values.
- Add docstring to `pypropprop/processed_property.py` with further detail about allowable kwargs for the function.

Fixed
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- Fix `DepricationWarning` from ambiguous boolean of empty iterable for default in processed properties.
- Fix bug in default value for default argument in processed properties.
- Fix failing test due to conflict between casting and iterable supported in processed properties.
- Fix bug causing incorrect operation of optional, iterable-allowed, no default processed properties.

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Fixed
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- Fix bug which raises error when using the `method` kwarg with `processed_property`, as reported in issue 48. Was caused by variables being out of scope in the function `apply_method` with `pyproprop/processed_property.py`.

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Added
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- A new `Options` utility class is now provided, as requested by issue 40. which allows groupings of options that are supported or unsupported. Default options for can be specified. Handles can also be provided and linked to options so that a dispatcher can be automatically generated for a set of options with associated functions/classes that are callable.
- String formatted is supported by Options, as requested in issue 41.
- Casting to Numpy ndarrays is now supported, as requested by issue 46.
- Processed properties can now be marked as "read-only", raising an `AttributeError` if they are tried to be set more than once.

Changed
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- `processed_property`'s setter now uses a dispatcher (which is greated during the initial function call) to build the setter method improving performance, as requested by issue 18.