Tablebase readonly mode

[Herren]

As suggested by @jodavies during the currently ongoing FORM workshop, I added an option
to the tablebase open command, allowing to open tablebases in read-only mode.
This allows to load tablebases in shared environments where not all users of the tablebases
have write permissions. In case the addto command is called on a readonly tablebase file,
an error is thrown and FORM terminates.

The syntax is simply:
TableBase "tablename.tbl" open, readonly

Some documentation on this feature is also added to the relevant manual file.

[coveralls]

coverage: 50.002% (+0.003%) from 49.999%
when pulling 7ed3e84 on Herren:tablebase_readonly
into 83e3d41 on vermaseren:master.

[jodavies]

Currently, if you attempt to open a non-existing tablebase in readonly mode, there is no error and the file is created. I think this case should print an error and terminate FORM.

[Herren]

Please have a look at the change. It should work now.

[jodavies]

Thanks, this works. Could you finally add test cases in check/features.frm for the failure modes, trying to add to an opened readonly table, and trying to open a non-existing table in readonly mode?

You can run multiple form scripts in a single test case like this: jodavies@9a5e42d