Add global const speed of light

[marcobonici]

CosmoCentral.jl/src/AngularCoefficients.jl

Lines 57 to 58 in c7aeba8

	c_0 = 2.99792458e5 #TODO: find a package containing the exact value of
	#physical constants involved in calculations

declare a global constant c0, with the correct value once for all.