Add function to get time range

Add get_time_range function to efficiently (for remote use) get the time
span of a given type of msid (content type).  Tom, I didn't see any
other relatively efficient way to do this, but if there is, let me know
and we can drop this change.

I ran the regression test on Windows (remotely) and everything matched except for daily DP_SUN_XZ_ANGLE data.  However, this data has always been different when I run the test on Windows, so I'm calling it good.

old: dp_pcad4             DP_SUN_XZ_ANGLE  daily  2e460bcd1995c2e58732ee82537b6709
new: dp_pcad4             DP_SUN_XZ_ANGLE  daily  75b7927e65638775e6c775292a6a22b5