approx_conditional() can be simplified

[jackmwolf]

approx_conditional() uses an equation that can be heavily reduced to estimate the conditional variance of a phenotype.

pcsstools/R/multiplication_estimation.R

Lines 416 to 417 in 7187ab8

	p_s2 <- (n * means[2]^2 + (n - 1) * covs[2, 2] - a * n * means[2] -
	b * (n * means[1] * means[2] + (n - 1) * covs[1, 2])) / (n - 2)

This can be reduced to:

p_s2 <- (n-1) * (covs[2, 2] - b * covs[1, 2]) / (n - 2)