Polymorphic field BCs and standing shocks

[JamesMcClung]

This is a mega PR with a messy history even after significant cleanup, so sorry about that. Here's a summary of the changes:

PSC Changes

• Stats dump interval can be ≤0 to disable it
• Fix read_checkpoint begin/endStep
• Fix integer division by 0 in taccum causing debug-mode crashes

FieldBcBase

• Abstract base class for polymorphic field boundary conditions
• Subclasses subsume the previous monolithic BndFields_ implementation
  • This greatly improves how radiating boundary waveforms can be specified
• Goal is to eventually subsume all BndFieldsBase types and possibly boundary-specifying enums
  • CUDA subclass remains

ConductingWall

• Handles conducting wall field BCs
• Automatically added by psc integrator based on boundary-specifying enum
• Dimension-agnostic implementation
• Add documentation

Radiating

• Handles radiating open BCs
• Templated by user-defined waveform provider
• Dimension-agnostic implementation
• Add documentation

SetupParticles

• Add VelocityBooster struct to facilitate velocity boosts
• Drifting Maxwellians are sampled in their rest frame and then boosted
  • The rest-frame sample is actually interpreted as a proper velocity to avoid crashing when a rogue particle's plasma-frame velocity exceeds 1; the correct thing to do would be to sample from a Maxwell–Jüttner distribution, but that's nontrivial

BoundaryInjector

• Experiment with "preacceleration", which seems to prevent Langmuir-like waves from building up at high-speed inflows
• Support upper boundaries
• SetupParticles sampling changes apply here, too

psc_shock Changes

• Fix parallel B component being 0
• Fix advective inflow when the domain corner isn't at the origin
• Fix advective inflow of turbulence when the initial background field isn't uniform
• Fix div B = 0 in 2D case
• Support relaxation method of initialization
• Param changes:
  • Add checkpoint-related params: checkpoint_filename, checkpoint_interval, n_checkpoints
  • Add stats_interval param
  • Add shock_method param ("wall", "relaxation", or "none")
  • Add θ_xz_deg and θ_Bn_deg
  • Add domain length params lx, ly, lz that override d* when present
  • Add downstream override params: r, T_e2, T_i2
  • Breaking changes to other params:
    • electron_mass -> m_e
    • ion_mass -> m_i
    • electron_temperature -> T_e
    • ion_temperature -> T_i
    • b_mag -> B_0
    • turb_dB -> dB
    • turb_correlation_length -> L_c
    • n_patches_* -> np*
    • Remove turb_dB^2
    • Remove b_angle_y_to_x_rad
    • Remove v_upstream_*
    • Remove mirror_domain