Behavior of default implementations

[schillic]

There are three ways we define fallback functions without implementation. I think we should use the same way consistently.

1. Sometimes we just declare a function:

LazySets.jl/src/Interfaces/ConvexSet.jl

Line 174 in 9fdf5e8

function σ end

2. Sometimes we throw an error:

LazySets.jl/src/Interfaces/ConvexSet.jl

Lines 793 to 795 in 9fdf5e8

	function isoperationtype(X::Type{<:ConvexSet})
	error("`isoperationtype` is not implemented for type $X")
	end

3. Sometimes we return false as conservative answer (of course this only applies to functions that return true/false):

LazySets.jl/src/Interfaces/LazySet.jl

Line 152 in 9fdf5e8

isconvextype(X::Type{<:LazySet}) = false