ENH: standard download warning routine

[jklenzing]

Is your feature request related to a problem? Please describe.
Some instruments don't have download routines, particularly during the ops phase. We could reduce code duplication by having a standard warning routine as part of the core package.

Describe the solution you'd like
A standard routine that could be attached to packages. Would basically warn users that downloads are not currently available.
May have the option to pass through a custom warning message.

Describe alternatives you've considered

• Keeping the code the way it is
• Automatically attaching this during instantiation if a download routine is not present should be actively aware of this

Additional context
Thought of during development for reach ops software. Would simplify pysat interface since downloads not yet available.