Add performance regression tests

[rafaqz]

Adding PhysicalConstants.jl completely killed performance here, due to some conversions to BigFloat which allocates.

We should catch problems like this in tests in future.

[mrke]

Shall we just use Unitful for most of the constants then? It has σ, R and gn and for atmospheric pressure we can just use u"Pa"(1u"atm")

[rafaqz]

PR incoming, I just converted them to float units

[rafaqz]

But note the point of the issue is more meta: we need to catch anything like this in the tests in future