Remove scalar indexing on `AbstractExpr`'s from atom formulation

[ericphanson]

As shown by #613 & #254 this leads to huge performance cliffs. Here is an audit of such issues in our current atoms:

ExponentialCone based issues

• [x] ExpAtom (fixed by #613): https://github.com/jump-dev/Convex.jl/blob/2a3b5202549d5c442bdafce01f157e5c168e3476/src/atoms/ExpAtom.jl#L37
• [x] EntropyAtom (fixed by #618): https://github.com/jump-dev/Convex.jl/blob/18d378ecfe1e2cbad31eacdc2378d41019659301/src/atoms/EntropyAtom.jl#L43
• [x] LogAtom (fixed by #618): https://github.com/jump-dev/Convex.jl/blob/18d378ecfe1e2cbad31eacdc2378d41019659301/src/atoms/LogAtom.jl#L37

These could all be handled just like #613. Perhaps we can find a way to reuse the code here rather than inlining near-identical implementations.

Other cases

• [x] AbsAtom in the complex case (fixed by #619): https://github.com/jump-dev/Convex.jl/blob/18d378ecfe1e2cbad31eacdc2378d41019659301/src/atoms/AbsAtom.jl#L28
• [x] GeoMeanAtom #621: https://github.com/jump-dev/Convex.jl/blob/18d378ecfe1e2cbad31eacdc2378d41019659301/src/atoms/GeoMeanAtom.jl#L43
• [x] QolElemAtom #620: https://github.com/jump-dev/Convex.jl/blob/18d378ecfe1e2cbad31eacdc2378d41019659301/src/atoms/QolElemAtom.jl#L37

[ericphanson]

🎉