Simsopt

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

This release introduces some additions and bug fixes for majority of modules in simsopt.

Changes introduced:
1. New class for Reiman magnetic field (see section 5 of Reiman, Greenside (1986), "Calculation of three-dimensional MHD equilibria with islands and stochastic regions", Computer Physics Communications 43 (157-167)) is added by **Rogerio**. This field has the advantage of allowing an exact calculation of the island width which can be used for island width optimization (example: https://arxiv.org/abs/2102.04497).
2. Faster Biot-Savart code by **Florian**
3. A simplified logging interface is introduced by **Bharat**. Logging can be enabled by using the following two lines in your driver script:
python
from simsopt import initialize_logging
initialize_logging(filename="simsopt.log") Filename option can be omitted in which case the default name is default.log

4. Some modules are rearranged.
5. Top-level imports for important classes are introduced.
6. Automatic linting script implemented by **Florian**. Before you submit your additions/changes to simsopt as PR, please run the `run_autopep` script. Requires autopep8 and flake8.
7. A new exception class to handle failures in objective function (credits to **Matt Landremann**).
8. Bug fixes.
9. Improved documentation.