Sudsoln

* Added `change_empty(array, old, new)` in `sudoku.py`.
* Erased `from sudsoln.candidate import *` and `from sudsoln.questions import *` in `__init__.py` because I think they should be accessed as submodules.
* Made `Sudoku` to guess `self.elements` whenever it is not specified (i.e. `None`); you don't need to specify it every time you initialize `Sudoku` puzzle unless error messages specifically ask you to do so.
+ Updated `tests.py` accordingly: added `test_init_detect_answer()`, `test_init_elements_error()`, and `test_init_empty_error()`.
* Wrote `import doctest; doctest.testmod()` in `candidate.py`

* Added `formatting.py` for a formatting of `questions.py`
* Added `test_solve_forcefully()` in `tests.py`
* Added `seed` and `quietly` arguments to `.solve()` and `.solve_forcefully()`
* Changed the numberings of `a_sta410_*` in `questions.py`, and added `q_sta410_testing` for unittest
* Removed `numpy.random` dependency from `.solve_forcefully()`
* Removed `sudsoln/tmp` directory and moved all the `.py` scripts out of it. `appearance.py` and `array.py` is now in `sudsoln` directory
* Rewrote `__init__.py`:
+ Added: `from sudsoln.candidate import *`; realized having this is more convenient
+ Added: `from sudsoln.questions import *`; the same reason
* Wrote `import doctest; doctest.testmod()` in `sudoku.py`

* Added `python_requres=">=3"` in `setup.py`
* Deleted `from sudsoln.candidate import *` in `__init__.py`; unnecessary.
* Deleted `solvers.py` in `sudsoln/tmp` directory; redundant.
* Made `Sudoku.elements` type-independent, i.e. i.e. it now accepts not only a set of strings, but also a set of integers, as well as a mixture of both
* Wrote a test on `.solve_logically()` with questions in `questions.py`

* Edited `tests.py`: instead of using `import sudsoln.candidate as candidate`, it is now `import candidate`

* Packaging failed; edited `__init__.py` in `sudsoln` directory.

* `__version__` variable directly defined in `__init__.py`, and `version.py` deleted.
* Added one unit test on Sudoku class
* Edited: from `from .candidate import *` to `import candidate` in `sudoku.py`
* Fixed typo in `to_sudoku()`: `(str, int, {objects})` => `(str, {objects}, str)`, and default has been defined for `elements` argument
* Moved `.py` scripts that are not currently a part of package: `array.py`, `questions.py`, `solvers.py`, `tests_candidate.py`, and `tests_sudoku.py`. These are currently located in `tmp` directory.
* Unified all tests into `tests.py`