Implement checks for sequences

[mwermelinger]

Create a file paddles/preconditions.py that will contain functions that check common preconditions. To start:

• Move function is_non_decreasing from test_sorting.py to this file and import it in test_sorting.
• Add 3 functions that check if a sequence is increasing, decreasing or non-increasing.
• Add a function that checks if all members of a sequence are comparable. Do this like checking for 'non-increasingness' but only stopping if a type error occurs (comparison not implemented).

Create a file tests/test_preconditions.py:

• Do not import pytest as this file won't use fixtures, only 'static' test data.
• Define a constant that is a list of strictly increasing sequences.
• Do the same for decreasing, non-decreasing, non-increasing and comparable sequences.
  • Each constant should include sequences that satisfy only that precondition. For example, NON_DECREASING should not include a sequence like [1,2,3] because that is a strictly increasing sequence.
• Write a test for is_non_decreasing. II consists of 5 loops, one per constant:

for sequence in INCREASING:
    assert is_non_decreasing(sequence)
for sequence in DECREASING:
    assert not is_non_decreasing(sequence)
...

• Do similarly for the other 4 functions.

Although this issue requires more effort than some of the others, it's relatively simple and thus a good first issue.