Define code style for j-Ecdar

The code base for j-Ecdar is not checked for adherence to any style, because we haven't defined a style for our code. As a result of this, multiple styles are used throughout the code, which makes it less consistent.

In another Java-based tool, we are using [Checkstyle](https://checkstyle.sourceforge.io/) to specify and automatically check the style of the code. There are style specifications, like [Google's Java style](https://checkstyle.sourceforge.io/google_style.html), which we can use as a basis for our project.