Change wavelength columns from 32-bit to 64-bit floats

Following discussions today on the drp-2d slack channel, please update datamodel.txt such that wavelength-related column types are changed from 32-bit floats to 64-bit double-precision floats.

The rationale behind this is to ensure that the column types are of sufficient precision to store wavelengths down to an accuracy corresponding to a better than 1 km/s radial velocity, ie better than 1/3e5 or 3e-6. A 32-bit float is not sufficient to store wavelengths to that level of precision.