renaming of fieldnames for quantities today

[mocalvao]

Dear all,

First, congratulations for the wonderful and well coded package!

May I make a small naive suggestion: that the parameters, for instance, h, Omega_m, Omega_r, etc be renamed to h_0, Omega_m0, Omega_r0. This is what they actually stand for (the values today) and, besides, in the future, it might be worth coding new functions which would provide the critical densities at an arbitrary time (or redshift).

Regards