Update date.py

[mailhot001]

remove the + 1 on the week RE match in date parser so that the week number match the front end displayed week number.

related to: #1805

[mailhot001]

@yagebu Here is my second try, I believe it matches the D3.js week parsing, I've tested it roughly (without edge case) and it is working the same as frontend.

[yagebu]

Thanks :) Can you add some test cases that show that this is now working correctly in cases where it didn't before?

[mailhot001]

I added a test case to show an error if ISO week format is used.
Also added a note in the help to explain the week numbering used, a month is a month, but week can be taken differently. The note should help users know how it work without checking the code and then checking the %W definition of strptime.
Or the equivalent definition used in D3.js.