Simsopt

This release adds adjoint methods (courtesy E J Paul), additional VMEC related residuals (courtesy M Landreman) and improved plotting options (courtesy C. Zhu).

This release introduces

1) The ability to compute fieldlines as well as particle trajectories using the guiding center approximation.
**Credits to Florian Wechsung.**
2) A quadratic-flux minimizing surface solver, defined as the minimizer of the objective function

f = int d^2 x (B \cdot n)^2/int d^2 x B^2

at fixed flux surface label (e.g., volume, area, or toroidal flux). This optimization problem can be solved using a
penalty formulation with LBFGS or with the SLSQP constrained optimization algorithm. Much of the code was
modeled after the Boozer surfaces.
**Credits to Elizabeth J Paul**

This release adds conda package recipe for simsopt, toroidal surface using the parameterization in Henneberg, Helander, and Drevlak ( arXiv:2105.00768 (2021), and bug fixes.

This release fixes bugs and reorganizes examples.

1. Earlier releases of simsopt required all optional packages to be installed. This release fixes issues in importing simsopt when optional packages are not installed.
2. The examples are reorganized into simple, intermediate, advanced, and stellarator_benchmarks.

This release eases the use of simsopt for end users. A docker container with simsopt and vmec preinstalled is available. It can be downloaded from DockerHub using the name hiddensymmetries/simsopt.

The coverage report is now avaialble and can be accessed from the codecov badge on the main README.me