Change pair spectrum: kz^2 -> 2(1-cos(kz)).

Results in somewhat lower Tc's and faster decline of x2 above Tc.

Also added skipPlots option to tempFluc/system_test.go to jump to
specific heat calculation in conjunction with loadCache.
interface to GSL calculation of Bessel I0
interface to GSL implementation of Levin u-transform
Test levin.go. Start collecting terms at the correct value.

Test requires 26 terms instead of the 13 terms shown in the example in the
GSL manual, and doesn't return an answer until 160 terms are available.
Not sure why.
implement magnetization calculation; untested
Swap finite-field x2 & magnetization to Expm1.

"more accurate than Exp(x) - 1 when x is near zero"

May want to also do this in zero-field x2 calculation.
fix index error in Levin_u; now uses expected number of terms
interface to GSL calculation of Bessel I1
implement pair energy for finite magnetic field
fix incorrect Be_field check in PairEnergy
add magnetization plots
add optional iteration to SolveD1Mu_hMu_b
add flag to switch pair spectrum from kz^2 to cos
fix test expected values to match cos spectrum