Sudsoln

* Added `by` and `start` arguments to `.solve_by_pairs()` of `Sudoku`
* Added `condition` and `deep` arguments to `.sieve()` method of `Appearance`.
* Added `sieve`, `condition`, and `deep` arguments to `.refine` method of `Candidate`
* Added `.keys()` and `.values()` method to `Appearance`
* Cancelled a plan to make `Sudoku` accept strings; it seems unnecessary because `to_sudoku()` function already does the job of transforming sudoku strings into `Sudoku`. Moreover, simplifying the type of `array` argument in `Sudoku()` seems important.
* Edited `__repr__()` of `Candidate`; dictionary will be displayed as if it is passed to `pprint.pprint()`
* Packaging done to check `.solve_by_hidden_pairs()`

* Packaging still failing in Python <= 3.6; moved `Appearance` and `Union` under `Candidate` and retried
* Removed `import appearance` and `import union` in `tests.py` as these objects are now moved to `candidate.py`

* Packaging failed in Python ver <= 3.6; modified `__init__.py` and retried.

* Added `.appearances()` and `.unions()` methods to `Candidate` object.
* Added `Appearance` and `Union` object that is used in `.solve_by_pairs()` method of `Sudoku` puzzle. They're written in `appearance.py` and `union.py` respectively.
* Adjusted `Candidate` object to have `elements` argument; `n` argument is completely deprecated and is computed via `elements`. All affected objects --- `Appearance`, `Union`, and `Sudoku` --- are accustomed to adapt this change.
* Rewrote the entire docstring examples of `Candidate` and `Sudoku` to pass doctest.
* Rewrote the test cases in `tests.py` to adapt the change in `Candidate`.

* Commented out `install_requires=['numpy']` in `setup.py`

* Added `Array` object, an array that is ALWAYS 2-dimensional:
+ supported attributes: `.show`, `.nrow`, `.ncol`, `.shape`, `.size`
+ supported methods: `.__eq__()`, `.__getitem__()`, `.__setitem__()`, `.__repr__()`, `.copy()`, `.flatten()`, `.itemset()`, `.reshape()`
* Added two tests in `tests.py`: `test_init_detect_typo_elements_none()` and `test_init_detect_typo_elements_specific()`.
* Added `TestArray(unittest.TestCase)` in `tests.py`.
* Changed the name of `array.py` to `sarray.py`
* Edited `test_init_detect_answer()` to test two features: answer-form detection and empty-specification irrelevance.
* Made `sudoku.py` completely numpy-free