Fixing #1271: Support for week number format

Adding support for '%U' and '%W' format sequences (similar to Open Group's strftime) in time series data. The functionality accounts for timezone options that may have been set. 

There was, however, some trouble getting the ticks to align with the start-of-the-week correctly, hence needed to broaden the threshold for aligning with 1st of the month to at least 15 days of tick size, and added 1-week and 2-week tick sizes which will not align to the 1st of the month.