Variables with missing descriptions

[clmould]

There are variables in the data_structure files that do not have any descriptions, these should be added. This list may not be exhaustive but I've tried to record all instances I've seen:

• All variables in cost_2015_variables.py

• Many variables in cost_variables.py - line 27 (c226) to line 201 (c22128)

• impurity_radiation_variables.py: imp_label, impurity_arr_label, impurity_arr_z, impurity_arr_len_tab, pden_impurity_lz_nd_temp_array, impurity_arr_zav

• neoclassics_variables.py: r_eff

• pfcoil_variables.py: nef, nfxf, ricpf, ssq0, sig_hoop, xind, ccls, ccl0, bpf2, vsdum, first_call, cslimit

• physics_variables.py:

  

• Many vars in power_variables.py

• stellarator_variables.py: first_call, first_call_stfwbs, powerht_constraint, powerscaling_constraint

• tfcoil_variables: sig_tf_cs_bucked, sig_tf_wp