Allow VisIt LAMMPS dump reader to read multiple time steps

The LAMMPS atom dump supports writing out multiple time steps. The VisIt reader supports reading these timesteps, and that works in VisIt proper, but ParaView was only reading the first time step.

The problem was that although the reader was properly filling the AVT metadata with the cycles, it does not properly override the GetCycles method of avtFileFormat. The VTK bridge was only looking at the data returned from GetCycles, so was missing the time information.

I don't know why this information is stored in two places, which place is used for what, or if this is a bug in avtLAMMPSDumpFileFormat. Regardless, the bridge works fine if you check for time information in the metadata if the reader itself does not return it.