Naming Scheme for 2N model

Following Rockendorf (1993), I would suggest to use the following abbreviations in SunPeek for the 2N model. Also, I would use T_s(x,t) instead of T_m(x,t) (i.e. s instead of m for the metal / solid part), because T_m can be easily confused with the collector array mean temperature. What do you think, @p.ohnewein

Parameter	Unit	Description
f_prime	-	collector efficiency factor, F' = u_int /(u_int + u1 + u2 (T_s − T_a ))
f_prime_c	-	constant collector efficiency factor F'_c = u_int /(u_int + u_1 )
tau_alpha	-	effective transmittance absorptance product
kb	-	incidence angle modifier for direct solar radiation
kd	-	incidence angle modifier for diffuse solar radiation
u1	W/(m2 K)	collector heat loss coefficient at T_s − T_a = 0
u2	W/(m2 K2)	temperature dependence of heat loss cofficient
u_int	W/(m2 K)	heat transfer cofficient between absorber and fluid
mc_s	J/(m2 K)	effective heat capacity of solid part of collector per collector gross area
mc_f	J/(m2 K)	effective heat capacity of collector heat transfer fluid per collector gross area

Measurement	Unit	Description
T_f	K	fluid temperature
T_s	K	solid (metal) temperature

cc / @p.z