source amplitude from HDF5 file

[stevengj]

The sources already allow you to specify an amp_fun that gives an arbitrary function of space for the amplitude. Might be nice to layer a higher-level function on top of this that reads in a grid of values from an HDF5 file and creates an amp_fun that interpolates into this grid. (Much like the epsilon_file feature.)

This could be put as a helper method into the fields object, I guess, a new method of add_volume_source, where you pass a filename and a dataset name instead of A. (You would still pass an amp parameter to allow you to multiply the values in the file by an overall scale factor.)