Interface to add to OfflineDataMapPoints so that they can serve up a datagram.
Interface implemented on a PamDataBlock which is able to provide Datagram type data.
A datagram is a set of data which summarises the real data content in quite course bins of about 10 minutes (though this may vary).
Generally, Datagram data look a bit like very course spectrogram data, except that they are designed to show data density as a function of frequency instead of energy density as a function of frequency.
They can be calculated automatically from many types of data and are stored in the binary index files for easy access when mapping data.
DatagramManager needs to return more than just an image - it also needs the start and end times of the data, which will have been extended out to interval boundaries.
Class to pass around a few bits of information about a datagrams scale