Discourage convert(Array{C}, img) in favor of broadcasting

[timholy]

Consistent with JuliaImages/ImageCore.jl#131

[timholy]

Several of these were not problematic in their original form, but my thought is that we should discourage anything that might encourage users to use convert(Array{C}, img).

Do you think we need to have a docs warning that convert(Array{RGB}, img) will result in an abstract eltype but that RGB.(img) will be concrete?

[johnnychen94]

Do you think we need to have a docs warning that convert(Array{RGB}, img) will result in an abstract eltype but that RGB.(img) will be concrete?

We can add it in #146. @mgautam98 do you want to continue that PR recently?

[mgautam98]

I can add it in #146. @mgautam98 do you want to continue that PR recently?

Yes, I want to continue. I will add this warning.