Allow easy way to specify mean/covariances for reference data that is not dependent on a time axis

[jbphyswx]

I am trying to run TurbulenceConvection.jl simulations whose reference should be observations. Although I need to specify the relevant time period for the output from the TC.jl runs, the observations have their own native values/variance that are a result of observational uncertainty and are not a function of time.

Luckily I have a profile dimension which we are using to determine means and covariances, so I am currently getting around this by pretending that profile is time and proceeding.

However, it would be nice to be able to natively specify perhaps means and covariances for the reference data without assumption of a time axis.