FieldSolver Classes: QuasiStatic

[ax3l]

Thamine @tdalichaouch mentioned in SciDAC-5 today that we could add a specialized QuasiStatic field solver interface to PICMI. Everyone agreed, let's go forward with a pull request.

Reference:

• https://github.com/tdalichaouch/PICMI
  • https://github.com/tdalichaouch/PICMI/blob/784be2766c6d8fecc1ff829e2c3994b4e9221411/picmi_two_bunch.py#L47-L50
• QuickPIC: https://github.com/UCLA-Plasma-Simulation-Group/QuickPIC-OpenSource
• QPAD: ...
• https://picmi-standard.github.io/standard/field.html