Added yield_normalised for better interface to yields

[aidancrilly]

Simple QoL addition, single function get normalised primary yields. Changing from:

# Scale from DT yield
Y_DT = 1.0   # DT neutron yield, all reactions scaled by this value
Y_DD = nst.yield_from_dt_yield_ratio('dd',Y_DT,Tion,frac_D=frac_D,frac_T=frac_T)
Y_TT = nst.yield_from_dt_yield_ratio('tt',Y_DT,Tion,frac_D=frac_D,frac_T=frac_T)

to:

# Sum of primary yields is unity
Y_DT,Y_DD,Y_TT = nst.yields_normalised(Tion,frac_D=frac_D,frac_T=frac_T)

[aidancrilly]

Closes #40