Utility function to compute integrated energy density

[astrolamb]

Is your feature request related to a problem? Please describe. Some models are bounded by BBN or CMB bounds on cosmological energy density. Having a switch to include these bounds in the prior function would be useful for constraining posteriors

Describe the solution you'd like

1. Create a utility function to calculate the integrated energy density across frequency for a model spectrum supplied with input parameters
2. Use that utility function in the log-prior function. If the integrated energy density is greater than a chosen bound, return -inf

Describe alternatives you've considered We could just take the posterior and remove samples that result in energy densities greater than the bound? How would these two methods change the posterior?

Additional context I'd like to compute the integrated energy density of the different spectra I'll be using to compare each other