Parameters for DFTB3

[awvwgk]

Clarify parameters for DFTD3 (probably 3ob SK set) and find original reference.

simple-dftd3/src/dftd3/param.f90

Lines 458 to 460 in 442fbec

	case(p_dftb3_df)
	param = d3_param(a1=0.5719_wp, s8=0.5883_wp, a2=3.6017_wp)
	! TODO: find reference