Add spin-asymmetry to exported output when appropriate

[bmaranville]

During an export operation, the reflectivity curves are exported as .dat text files. It has been requested that, when the probes are polarized, we also export the spin asymmetry as a .dat file (with the correct error bars) for plotting.

[pkienzle]

We should be using the uncertainties package for error propagation since we have correlations between the uncertainty in the numerator and in the denominator. This is all the more reason that we would want to produce spin asymmetry data as a separate output.

[bmaranville]

We are currently calculating it with what I think is the correct error propagation, hard coded instead of using the uncertainties package (it never changes)

[pkienzle]

Okay, verified:

from math import sqrt
from uncertainties import ufloat as U

x, y = U(3, 0.1), U(2, 0.2)
print(repr((x-y)/(x+y)))
# => 0.2+/-0.05059644256269408

# our formula
print(2*sqrt((x.n*y.s)**2 + (y.n*x.s)**2)/(x.n+y.n)**2)
# => 0.05059644256269408

# if numerator and denominator were independent the answer is different
xp, yp = U(3, 0.1), U(2, 0.2)
print(repr((x-y)/(xp+yp)))
# => 0.2+/-0.04560701700396553