transit_demo.cfg makes eclipse spectrum

[iancrossfield]

Minor issue: the default 'transit_demo.cfg' file generates an emission ("eclipse") spectrum instead of a transit, because it has "solution" set to "eclipse."

[rychallener]

This is because "transit_demo" refers to the program Transit, rather than transit geometry, so this demo is for the program rather than the transit mode. The real issue here is that our radiative transfer code is confusingly named Transit, but I don't think that's something we have the liberty to change.

[aj-foster]

(See #1) We should probably revive that discussion.