Computes virial coefficients for a helium pair potential. Integrand is computed with
Use semiclassical boolean to change pair potential to the quadratic Feynman-Hibbs potential.
Use calcApprox boolean to calculate quantities with the approximate potential.
Use subtractApprox boolean to compute the correction to that approximate potential.
If determining which option is most efficient via short calculations to estimate standard error,
maintain a 50-50 split of steps between reference and target during data collection.