Reduce code duplication

[giordano]

Following up #24, I've changed the signature of

Z(c::AbstractCosmology, z::Real; kws...)

to

Z(c::AbstractCosmology, z::Real, ::Nothing; kws...)

then removed methods with a single redshift and added nothing as default value of the second redshift for the methods with two redshifts.

This is an implementation detail that shouldn't matter for the users, the API is identical, but allows us to remove almost identical methods.

CC: @astrozot.

[astrozot]

Sounds good!