fix(#47): count step values from the field minimum

[Prafyl]

Fixes #47.

*/n was checking target % n == 0, which only works when the field starts at 0. Day of month and month start at 1, so */6 on day of month matched 6, 12, 18, 24, 30 instead of 1, 7, 13, 19, 25, 31, and */3 on month gave Mar, Jun, Sep, Dec instead of Jan, Apr, Jul, Oct.

_parse_arg now takes a min_value and checks (target - min_value) % n == 0. is_now passes min_value=1 for the dom and month fields. Minute, hour and day of week start at 0, so they keep the default and are unchanged.

Two existing assertions had to change. Like you said in the issue, test_dom.py and test_month.py both use June 18th, and 18 is divisible by 6 and June is divisible by 2, so they were passing on the old behaviour:

• test_dom.py: * * */6 * * is now False on the 18th
• test_month.py: * * * */2 * is now False in June, * * * */5 * is now True

Added test_dom_step and test_month_step, which use dates on and off the step and check that */n gives the same answer as the equivalent 1-31/n and 1-12/n.

Coverage stays at 100%, and flake8, pylint and mypy are clean.

[kipe]

Thanks, looks good 👍