Add capability to read exodus files with time-varying data

[jvwilliams23]

Hi,

I have added code to read exodus files with time-varying data.

A field "a" that contains "values" of length T previously returned values[0]. Now, if len(values) > 1, new fields are created such as ["a_time0", "a_time1", ..., "a_timeT"]. This has been implemented for point data and cell data.

I added a unit test for cell_data (not time-dependent), and also for time-varying point_data. The latter was a bit trickier, I couldn't find out how to create time-varying data in the in_mesh object. So instead, exodus.write, will write the same value for each step. This is just so the unit-test can check that something is being written. This is probably not the best thing to do, but it seems the data cannot be viewed in paraview anyway, so we could consider adding a warning to use xdmf for writing time-dependent data.

Thanks for developing this great tool.
Josh