Add infrastructure

[florianm]

• Close Add infrastructure #53
• Add devcontainer, vscode settings
• Add pre-commit, disable invasive hooks, enable for devcontainer
• Add pkgdown site, customise theme
• Update DESCRIPTION
• Add GPL-3 license
• Add contributing guide
• Add GH issue and pull request templates
• Add GH actions for R CMD check, codecov, pkgdown
• Add README.Rmd
• Delete docs/, replaced by GH action pkgdown building and publishing the site to a separate branch and hosting via GH pages.
• Delete travis CI, replaced by GH action R CMD check
• Changes to existing codebase: comment out two tests that pass in codespaces but fail on Gh actions, format tests, remove obsolete lines from tests to resolve warnings, Roxygen-markdown converted some latex markup to markdown in function docstrings in R and man/.
• NOTE: badges are formatted to the upstream repo carlopacioni/vortexr. They have been tested in the fork florianm/vortexr and might only start working upstream after merge and adding the carlopacioni/vortexr CODECOV_TOKEN.

Checks

The included new GitHub actions workflows can be reviewed in the WIP PR at florianm#1. This PR contains the same changes, but rebased for clarity and simplicity.

Actions after merge

These actions require admin access to carlopacioni/vortexr which currently only @carlopacioni has.

• Enable codecov for carlopacioni/vortexr, add CODECOV_TOKEN as repository secret to carlopacioni/vortexr
• Disable vortexr on TravisCI and readthedocs.
• Enable the commented-out pre-commit hooks which will format and style the codebase. This will generate a lot of no-op changes which would look nicest in the git blame history if the author @carlopacioni did that himself.