Add better time-stamp extraction

[tlambert03]

Time stamp info is currently scattered in different places depending on the nd2 version.

While there is some information on time loop interval in reader.experiment

the actual timestamp for newer files timestamps for each frame is at

channels = nd2file._rdr._frame_metadata(i).get('channels')
timestamp = channels[0]['time']['relativeTimeMs']

for legacy readers, it's likely at

nd2file._rdr._get_xml_dict(b"VIMD", n).get('TimeMSec')  # where n is the frame number

this should be unified for the public API

[tlambert03]

ND2File.events() is likely the best timestamp info now