update .gitignore

Generate a more appropriate .gitignore file for the project

https://git-scm.com/docs/gitignore

I have been using this page in order to generate them automatically

https://gitignore.io/

The LaTeX part is good so far, but we will need at least to extend it to octave files as well. You can see from the header which keywords I used so far:

latex,linux,windows,macos,vim,sublimetext

You can visit

https://www.toptal.com/developers/gitignore?templates=latex,linux,windows,macos,vim,sublimetext

in order to extend the keywords to what we further need.