Compute parallel and perpendicular temperatures for the shock demo

[henry2004y]

Show that temperature only increases in the perpendicular direction.

[henry2004y]

We may have some errors in computing the thermal velocity. There are many different definitions of thermal velocity. Which one should we follow? https://en.wikipedia.org/wiki/Maxwell%E2%80%93Boltzmann_distribution#Typical_speeds
https://en.wikipedia.org/wiki/Thermal_velocity

Our current implementation

TestParticle.jl/src/sampler.jl

Lines 54 to 59 in eb46011

	function sample(vdf::Maxwellian, nparticles::Int)
	sqr2 = typeof(vdf.uth)(√2)
	# Convert from thermal speed to std
	σ = vdf.uth / sqr2
	v = σ .* randn(typeof(vdf.uth), 3, nparticles) .+ vdf.u0
	end

uses the definition here. I made a mistake in converting from Maxwellian distribution to Normal distribution. It is known as the most probable speed in 3D. We should clarify it and probably check the dimensionality before sampling.

Creating new distribution and sampler: https://juliastats.org/Distributions.jl/stable/extends/