338 support rendering into latex files

[JavierCladellas]

• closes Support rendering into LaTeX files #338
• closes optimize slow antora compilation time #265

Done in this PR:

• Support downloading remote images
• Cascade render method through reports
• Simplify figures code
• Plots are rendered using a JSON file stored under attachments. HTML content is heavily reduced. Data is always treated as asset.
• Add templates for rendering into LaTeX. Basically we have JSON -> adoc and JSON -> LaTeX. Text syntax in json mostly follows asciidoctor syntax, which is translated into latex (e.g. bold * * --> \textbf{})
• Add latex download button to reports.
• Added tests