Add LibreOffice to Build Servers

[smk78]

We have established the viability of using LibreOffice Writer 'flat XML' (.fodt) files with GitHub version control as a platform for writing the new SasView Tutorials.

Users will want these Tutorials in easily accessible, portable, formats. Certainly html, pdf (so they can print them), and possibly eBook.

We therefore need to be able to build these formats from the .fodt

Our initial idea had been to do this with PanDoc… until we discovered that whilst PanDoc supports the OpenDocument Text (.odt) format, it doesn't support .fodt! So we might just as well do the delivery format conversions with LibreOffice.

LibreOffice can be run from the command line. Invoke as {pathto}soffice.

See also: ​https://help.libreoffice.org/Common/Starting_the_Software_With_Parameters

[smk78]

Also see #934