Tables: Support for references to remote tables

Tables: Support for references to remote tables

Formulas can now refer to fields and regions in other tables, be it in
the same file or even in different files.

This also opens the door for more interesting R support, because
arbitrary tables in a file can now be accessed with appropriate
commands.

Finally, this commit also introduces a convenience for creating
references in tables.  For the purpose of references, it will always
be *assumed* that there is another hline at the end if each table.  So
for example in a table that has only a single hline, to separate
header from data, you can now access all of column two with
"@I$2..@II$2".