Improve accuracy of quartic_roots

[NAThompson]

Previously, we took a square root, and then squared it later in the computation. In Issue #1055, we observed that this caused enough loss of accuracy to prevent finding real roots.

Use the original value, instead of squaring the square root.

[mborland]

This looks good to me. The only error in CI is a personal favorite:

====== BEGIN OUTPUT ======
Running 1 test case...
Default hardware concurrency = 2
terminate called after throwing an instance of 'std::domain_error'
  what():  You have done something wrong.

Testing that a reasonable action is performed by the Monte-Carlo integrator when the integrand throws an exception on type float
unknown location(0): fatal error: in "naive_monte_carlo_test": signal: SIGABRT (application abort requested)
naive_monte_carlo_test.cpp(457): last checkpoint: "naive_monte_carlo_test" test entry

*** 1 failure is detected in the test module "naive_monte_carlo_test"

EXIT STATUS: 201
====== END OUTPUT ======

[NAThompson]

@mborland : Someday I'll fix that one . . .

@wztzjhn: Fixed on develop.