Check format using github actions, implement the /format command

[exaexa]

This implements the functionality required for finally closing #322 . Additionally this implements simple whitespace error checkers for eof newlines and trailing spaces.

I decided against opening purely technical shortlived PRs by the bot -- the /format command literally just pushes a single commit that can be erased if people don't like it. Also, the opening of PRs causes substantial clicking overhead (and a lot of noise in my mailbox), and seems more fragile.

The code needs to reach the default branch before the /format command will work.

Demo follows below.

PS. the PR includes a tiny change in the internals of GeckoModel that I saw while fixing the space errors and didn't want to make yet another gecko PR about it.

Confirmation that formatting is ok

Message about formatting deficiencies

Formatting command, success message and the commit

[github-actions]

🟥  Commit a294a6e requires formatting!

Required formatting changes summary:

 src/analysis/gecko.jl                 | 2 +-
 src/base/types/wrappers/GeckoModel.jl | 6 +++---
 test/analysis/gecko.jl                | 2 +-
 3 files changed, 5 insertions(+), 5 deletions(-)

[github-actions]

🟥  Commit f8e52a6 requires formatting!

Required formatting changes summary:

 test/analysis/gecko.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

[github-actions]

🟢  Commit 84736d6 is formatted properly.

[codecov]

Codecov Report

Merging #615 (a294a6e) into develop (581400d) will not change coverage.
The diff coverage is n/a.

❗ Current head a294a6e differs from pull request most recent head 84736d6. Consider uploading reports for the commit 84736d6 to get more accurate results

@@           Coverage Diff            @@
##           develop     #615   +/-   ##
========================================
  Coverage    90.29%   90.29%           
========================================
  Files           78       78           
  Lines         1876     1876           
========================================
  Hits          1694     1694           
  Misses         182      182           

---

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[github-actions]

🟢  Commit cb98224 is formatted properly.

[github-actions]

🟢  Commit 02ff9d9 is formatted properly.

[github-actions]

🟢  Commit 0bd69ec is formatted properly.

[github-actions]

🟢  Commit 9d47ecf is formatted properly.

[laurentheirendt]

looks awesome 🎉