`psy6`: add hint for failure to parse

[luke-kiernan]

We should add a hint such that if someone tries to do sys = System("foo.raw") or similar (matpower), they are told how to properly parse such a filetype. Definitely before release, but even now, such an addition would be greatly appreciated.

[jd-lara]

This is a PFFP issue, the interface to read directly from raw will be gone in psy6

[luke-kiernan]

But for UI reasons, the hint should still land here imo. Otherwise...what's the alternative? Because right now, it's a multi-repo scavenger hunt. (I now have used PFFP to turn my raw file into a PowerModelsDict, but I'm still trying to figure out how to turn that into a PSY.System. I've checked PFFP and PSB so far but no luck.)

[jd-lara]

In psy6 we are addressin that by having a specific load from file function that should have this documentation. but there is one single load from file function here.

We definetly need to document PFFP and PTDP but that's separate. My point is that this an issue to be opened in the parser repos not in PSY

[luke-kiernan]

My point is that users are going to try the usual thing. When it doesn't work, we should check if they're passing a .raw file and if so, provide explicit instructions or a link to a documentation page. Similar to how programming languages will provide hints when you inevitably mix up push and append.

We should probably also do something similar for the unitful getters/setters.

edit: at the moment, what works is

using PowerFlowFileParser, PowerSystemCaseBuilder
sys = PowerSystemCaseBuilder.make_system(PowerModelsData("foo.raw"))