Add field ionization

[branmedi]

Add field ionization physics to VPIC:

• Calculate the ionization rate (multiphoton ionization, ADK ionization, and BSI ionization) based on the interpolated E-field
• Compare ionization probability and uniform random number to decide if a given ionization event occurs.
• Add functionality to check for multiple ionization events in a single timestep.
• Change the particle's charge to make it the appropriate ion.
• Add a macro electron with the appropriate weight.
• Account for energy conservation due to energy loss from the field.

[branmedi]

Restarts are working.