dateType

[Danielle et Jean Thierry-Mieg]

I hope it is useful
To insure better compatibility with OPM and to the request of Dertlef
i introduced a new basic numeric type in acedb
_DateType, to be used like _Int and _Float in models
the whole code nows a bout it.

It is internally a time_t and arithmetics can be
preformed on it in particular the query can find the max min average
off a set of dates, I am not sure if average works well by the way on 
dates.
The input format is the "%c: unix format described in man ctime
sois a string of char but it is internally converted to a time_t number

This is part of the soon to be released ace.3-1