New tutorial page

[jvsguerra]

Main changes in pkg:

• Fixes details stated in first post of Issue;
• Add Tutorial page;
• Format R code according to styler R package.

Minor changes in pkg:

• Update .gitignore to GitHub R template;
• Remove ignorable files from repository;

Suggestions:

• Change "Run KVFinder" to "Run Cavity Analysis" (agnostic to inner software);
• Change "Get latest results" to "Retrieve results";

TODO:

• Review grammar in new text;