…1514)

* [pyobj] rename open-row/open-cell to open-row-pyobj/open-cell-pyobj

* [open-cell] add 'zo' to open file or url from path in current cell

Closes #1512

* [tests] clean up load-zip.vd

The original currency conversion API data.fixer.io redirects already to apilayer.com and I couldn't find
a way to create API keys for the former data.fixer.io API, so I updated the url and passing of parameters. Especially the api key is passed now as header instead as a url param what's certainly reasonable for security.

apilayer blocks urllib requests as bot requests (for at least for the free account).  apilayers blocks urllib requests even with different user-agents.  So, the solution is to add more headers like described in
https://stackoverflow.com/questions/13303449/urllib2-httperror-http-error-403-forbidden

(i don't know what I am doing, just following the instruction)

Potentially tests/ will not exist in a context, better to save in
current folder.

Closes #1499

IMHO a guarding of a sheet should be reverseable,
so added this simple command.

Not sure if we should show a status,
when guarded it's obviously we show,
but I decided against showing a status,
as unguarded is the normal behaviour and see no reason to differentiate
in the status between never guarded or once guarded and then unguarded.

There was a File > Guard menu item so far.
Didn't want to add a 2nd File > Unguard item for a probably very seldom
used functionality.

So changed it to an on/off selection as
File > Guard > on
               off
what I think is still convenient and very self explainary.

Now you can do:

@VisiData.before
def run(vd, *args, **kwargs):
    ABC()

to execute ABC() before the rest of vd.run().  Similarly @after will run
after (but return the value from the original function).

Part of #1513

Part of #1513