vmec_fieldlines: reduced number of iterations for root finding

[landreman]

PR hiddenSymmetries#243 looks good now. The little difference in this commit is the x0 for root finding is changed from theta_guess - 1.0 to theta_guess, which gives a modest reduction in the number of iterations required for root-finding in the VmecFieldlinesTests tests. The scipy.optimize.newton routine being used now does not require the root to be bracketed, so the algorithm converges to the root faster by starting with this more accurate guess.

BTW we can ignore the failing "Docker container" CI - it fails in forks because a password is not available.

[rahulgaur104]

Thanks, Matt! I don't think I can merge this commit with the hiddensymmetries/simsopt page.
This is what I get

[rg6256@stellar-intel simsopt]$ git push origin master
Username for 'https://github.com':
Password for 'https://rahulgaur104@github.com':
Enumerating objects: 90, done.
Counting objects: 100% (86/86), done.
Delta compression using up to 48 threads
Compressing objects: 100% (69/69), done.
Writing objects: 100% (69/69), 17.91 KiB | 5.97 MiB/s, done.
Total 69 (delta 47), reused 0 (delta 0), pack-reused 0
remote: Resolving deltas: 100% (47/47), completed with 12 local objects.
remote: error: GH006: Protected branch update failed for refs/heads/master.
remote: error: 1 review requesting changes by reviewers with write access.
To https://github.com/hiddenSymmetries/simsopt.git
! [remote rejected] master -> master (protected branch hook declined)
error: failed to push some refs to 'https://github.com/hiddenSymmetries/simsopt.git
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