Calculate conversion factors from constant names for CDF files

[smangham]

For example: To get from V^2 m^-2 Hz^-1 to W m^-2 Hz^-1 we multiply by ε0 * c. Currently, these values are stored as numbers in the CDF configuration files. Ideally, we would use the constants themselves. So:

"conversion": "* eps_0 * c",

Instead of:

"conversion": 0.002654418728,

Using the names of the Astropy constants, and looking them up. This is a bit convoluted to implement.