Symmetria

FEATURE:
- `symmetria.Permutation`: add `degree` method
- `symmetria.CyclePermutation`: add `degree` method
- `symmetria.Cycle`: add `degree` method
- `symmetria.random`: add random module to create random permutations

ENHANCEMENT:
- `symmetria.Permutation`: change how the sign is computed
- `symmetria.Permutation`: add `__new__` method to the class
- `symmetria.CyclePermutation`: add `__new__` method to the class
- `symmetria.Cycle`: add `__new__` method to the class

FEATURE:
- `cli`: add (minimal) command line interface

DOCUMENTATION:
- add documentation for command line interface

FEATURE:
- `symmetria.Permutation`: add `describe` method
- `symmetria.CyclePermutation`: add `describe` method
- `symmetria.Cycle`: add `describe` method

DEPRECATED:
- `symmetria.Cycle`: delete method `__pow__` as was not consistent
- `symmetria.Cycle`: delete `NotImplementedError` for `is_regular` and `is_conjugate`
- `symmetria.CycleDecomposition`: delete `NotImplementedError` for `__int__`

DOCUMENTATION:
- `README.md`: improve section quickstart
- `quickstart.rst`: improve examples

FEATURE:
- `generators`: add structure to generate permutations
- `generators`: add `lexicographic` generator
- `generators`: add generator following `Heap`'s algorithm
- `generators`: add generator following `Steinhaus-Johnson-Trotter`'s algorithm

FEATURE:
- `symmetria.Permutation`: add `ascents` method
- `symmetria.CyclePermutation`: add `ascents` method
- `symmetria.Permutation`: add `descents` method
- `symmetria.CyclePermutation`: add `descents` method
- `symmetria.Permutation`: add `exceedances` method
- `symmetria.CyclePermutation`: add `exceedances` method
- `symmetria.Permutation`: add `records` method
- `symmetria.CyclePermutation`: add `records` method

MAINTENANCE:
- `tests`: Refactor. Now the code is more concise, and it is easier to add a new test.
- `symmetria.elements._base.py`: Simplify the base class for elements object.

FEATURE:

- `symmetria.Permutation`: add `is_regular` method
- `symmetria.Cycle`: add `is_regular` method
- `symmetria.CyclePermutation`: add `is_regular` method
- `symmetria.Permutation`: add `inversions` method
- `symmetria.Cycle`: add `inversions` method
- `symmetria.CyclePermutation`: add `inversions` method
- `symmetria.Permutation`: add `image` property
- `symmetria.Permutation`: add `__pow__` method
- `symmetria.Cycle`: add `__pow__` method
- `symmetria.CyclePermutation`: add `__pow__` method