Allow no composition derivatives in get_ATrhoXi_runtime

[ianhbell]

See

teqp/include/teqp/derivs.hpp

Lines 261 to 279 in ecfcd29

	#define get_ATrhoXi_runtime_combinations \
	X(0,0,1) \
	X(0,0,2) \
	X(0,0,3) \
	X(1,0,1) \
	X(1,0,2) \
	X(1,0,3) \
	X(0,1,1) \
	X(0,1,2) \
	X(0,1,3) \
	X(2,0,1) \
	X(2,0,2) \
	X(2,0,3) \
	X(1,1,1) \
	X(1,1,2) \
	X(1,1,3) \
	X(0,2,1) \
	X(0,2,2) \
	X(0,2,3)

For you @Matthew-Boyd

[Matthew-Boyd]

Could you enable X(0,0,0) too? This would simplify calculating the reduced Helmholtz using dimensionless parameters by not having to back-calculate them from T and rho (via casts to get Tr and rhor).

[ianhbell]

Yeah, one sec. I vaguely recall there was a reason why that was a bad idea, but I don't remember what it was. Maybe it was elsewhere now that I think about it.

[ianhbell]

I needed a separate approach for 0,0,0, but I sorted it. You should be good to go