PDF process: command-line change to mn2pdf stack and heap space

[ronaldtse]

We need to expose the thread stack and heap space memory limits of mn2pdf for conditions like this:

• https://github.com/metanorma/iso-2533/issues/20

Where the 2GB limit to heap space does not allow PDF generation to complete.

I propose that we use something like:

--java-heap-min {defaults to 512m}
--java-heap-max {defaults to 2g}
--java-threadstack-max {defaults to 5m}

And this goes into the mn2pdf-ruby library as well.

Depends on:

• Allow configuration of mn2pdf stack and heap space  mn2pdf-ruby#39

[Intelligent2013]

From https://github.com/metanorma/iso-2533/issues/20:

We need to document when additional heap space is needed too.