transfer n_digits_data_round to CapacityExpansion.jl

In `run_clust.jl`

I think the data rounding is an issue that comes up in the CEP, so it should be handled there. There may be more generic applications of ClustForOpt where this hardcoded value adds to complexity. 
I am currently rewriting parts of run_clust.jl. After I push to dev, let's aim at this issue. 