data transfer function and uncertainty

[iej2106]

Obtaining Transfer function T(k) data + uncertanty

1. Loading lya: k, Pk to data object -> P(k)nCDM
2. Loading P(k)cdm (from class) for corresponding k in lya data.
3. Calculate uncertanty in T(k) from data uncertanty in lya data
4. Return data: k, T(k), T(k)_unc

[timothydmorton]

@iej2106 if you accept the invitation to the usc-cosmolab organization, I can officially assign you to #5

[timothydmorton]

See the comments; if you push edits to the same branch from which you made the PR, it will update.

[timothydmorton]

Looks great- only thing I would suggest is that it's more standard practice to comment above the code that's about to come rather than below. Switch that up and then I'll merge this in.