Updates to generate aggregate DX curves

[aowabinr]

Updates for generating aggregated curves for DX (wt-avg and nn-wt-avg):

• Changing ref_cap as var_name -> ref_gross_cap
• accommodating additional out_vars (i.e. eir-f-ff and cap-ff)
• changing threshold for compute_grad (i.e. tolerance on how much we can accept anomalous gradients) changed from 1e-5 to 0.02.

Note: In the future we need to make target variables for computing the weighted average (ref_gross_cap, part_eff, full_eff) as input arguments to SetofCurves class

[lymereJ]

Tests were failings, I had to make some changes. @aowabinr - please review and merge if that looks good to you.