simsopt v0.2.0

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:

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.