Examples for 3D space charge benchmarking

- Modified the initial beam size in the IOTA lens benchmark example.
- Added 2 benchmarks of 3D space charge for initial testing.
- Add documentation for 2 benchmarks with space charge.
- Add a benchmark example with space charge and periodic s-dependent focusing.
- Added an s-dependent example using a Kurth beam without space charge.
- Modified tolerance for IOTA lens benchmark example.
  Reduced tolerance to account for smaller initial beam size and
  improved preservation of invariants of motion.
- Modified tolerances of space charge examples to allow CI tests to
  pass when space charge is not active.

- Modified tolerance for space charge examples.
  These should fail unless space charge is turned on.

examples/kurth/input_kurth_10nC.in

@@ -0,0 +1,37 @@
+###############################################################################
+# Particle Beam(s)
+###############################################################################
+beam.npart = 10000
+beam.units = static
+beam.energy = 2.0e3
+beam.charge = 1.0e-8
+beam.particle = proton
+beam.distribution = kurth6d
+beam.sigmaX = 1.2154443728379865788e-3
+beam.sigmaY = 1.2154443728379865788e-3
+beam.sigmaT = 4.0956844276541331005e-4
+beam.sigmaPx = 8.2274435782286157175e-4
+beam.sigmaPy = 8.2274435782286157175e-4
+beam.sigmaPt = 2.4415943602685364584e-3
+
+
+###############################################################################
+# Beamline: lattice elements and segments
+###############################################################################
+lattice.elements = constf1
+
+constf1.type = constf
+constf1.ds = 2.0
+constf1.kx = 1.0
+constf1.ky = 1.0
+constf1.kt = 1.0
+
+
+###############################################################################
+# Algorithms
+###############################################################################
+algo.particle_shape = 2
+algo.space_charge = true
+
+amr.n_cell = 40 40 32
+geometry.prob_relative = 1.0