Silently ignore the '--no-rfi' flag when aoflagger feature is disabled

[Mouriyan-Rajendran]

When "--no-rfi" flag is passed to Birli when the aoflagger is not enable, Birli fails with the below error

error: Found argument '--no-rfi' which wasn't expected, or isn't valid in this context
If you tried to supply `--no-rfi` as a value rather than a flag, use `-- --no-rfi`

Expectation:
As "--no-rfi" is used to ignore the rfi flagging , which is same as aoflagger being disabled, Birli can silently ignore the "--no-rfi" flag is it passed while ao flagger is disabled.

[d3v-null]

I'm not sure if this is really an issue.

If birli is not compiled with aoflagger, then currently, the user has no reason to expect that it would do any RFI flagging, and so the --no-rfi flag shouldn't have a reason to exist.

I do intend to add other RFI options to Birli in the future, so I can fix this when that happens.

[Mouriyan-Rajendran]

From ASVO's perspective, when a user requests "No RFI flagging" ASVO adds "--no-rfi" flag to the biri command. ASVO doesnt know whether the birli is complied with aoflagger or not, so it expects birli not to fail becuase it added a flag though it is not required.

[gsleap]

As Mouriyan points out, the users of Birli can be different to the builders of Birli, so I think the following three suggestions have merit (but feel free to take or leave them!):

1. As per the original issue- --no-rfi when the aoflagger feature is not enabled should not cause a fatal error- maybe emit a warning that since the aoflagger feature isn't enabled, that this setting has no effect.
2. When aoflagger feature is NOT enabled, birli --help should explicitly state that there will not be any RFI flagging. I've been testing and playing with Birli on my machine now for years and never even realised that aoflagger wasn't enabled- if you do birli --help then there are lots of "flagging" options (ofc this is not RFI flagging but with a quick glance there is nothing to say that RFI flagging isn't enabled).

The above image is what the help looks like WITH aoflagger. It would be great if it showed under the same heading "This feature has not been enabled" or something. Right now it just omits the aoflagger section, so it's quite ambiguous if you were not the builder of the binary.

The reason the above 2 suggestions are important is because there is no command line option for the user to turn ON RFI flagging. Basically that is determined by the state of the aoflagger feature flag, set by the builder. So when you run Birli and don't mention either way if rfi flagging is on or off, there is no way for the user to easily tell if they expect rfi flagging to be done or not. Hopefully this makes sense?