Add simplified Ar cooling curves

[mikekryjak]

Impurity radiation dominates detachment in reactor-class devices but it's a very nonlinear process partially because of the nonlinearities of the cooling curve:

• Both the LHS and RHS slopes are curved
• RHS slope tapers off slower than the LHS slope
• RHS slope has a large "shoulder", a region of non-zero radiation at higher temperatures which is also a complex curve

All three features are present in curves of N, Ne, Ar and C to varying degrees. To study the impact of this, I prepared two simplified curves for Argon. fixed_fraction_argon_simplified1 takes out the curvature from the slopes but preserves their levels:

fixed_fraction_argon_simplified2 takes out the RHS shoulder:

fixed_fraction_argon_simplified3 takes out the LHS - RHS asymmetry:

Note that there is an additional point in the LHS limit to smooth out the transition to no radiation in the low temperature limit. Sharp transitions there have led to long runtimes in the past.

• Add the first two simplified curves
• Test
• Add the third curve
• Test
• Write docs

[mikekryjak]

@bendudson ready for review