Added footnotes to STILTS syntax

Added URLs as footnotes (using also a footnote separator mark) to the
STILTS syntax for selecting columns and rows.
Added mechanisms for supplying previous steps

Alternatively, the parameter can be supplied from a previous step in
the workflow.
Updated PDF
Added footnote linking to IVOA Documents
Added space between citation (Deelman 2005) and workflows
s/This/This understanding/ in executable graph paragraph
s/key in order to/vital to/ after SEPUBLICA2012 reference
allows providing free-text > allows the software designer to add free-text
s/not well solved/unresolved/
Corrected links to STILTS syntax for LaTeX

~mbt > \~mbt
Another correction for taking into account that \~m puts a tilde on top of m
Updated PDF
Changing \textbf{acknowledgements}

Includes changing \textbf{acknowledgements} a
\begin{acknowledgements}\end{acknowledgements}
Removed \textbf

Had also to break a paragraph to allow for consistent positioning of
references.
Merge pull request #1 from AMIGA-IAA/remove-textbf

Remove textbf
Merge pull request #3 from stain/votable-figure-ref

Figure reference for VOTable rendering
Increased width of WfDiagram

Includes updated PDF
Fixed small typo: s/Fig ~\ref/Fig.~\ref/