- added new functions max2/min2 reutrn NA instead of Inf/-Inf
- the 'to' argument in date_from function supports vector
- added an inner function dat_filter to filter data set by given date range
- fixed a bug in the date_num function
- added a new function date_num to convert date to number
- fixed a few bugs
- added cols_type and cols_const function
- fixed a bug in date_from function
- Fixed a few bugs
- Initial version