Move ConditionalMeasure to MeasureBase

[cscherrer]

Currently in conditional.jl we have

using NestedTuples: lazymerge

struct ConditionalMeasure{M,C} <: AbstractMeasure
    parent::M 
    constraint::C
end

Base.:|(μ::AbstractMeasure, constraint) = ConditionalMeasure(μ, constraint)

@inline function logdensity_def(cm::ConditionalMeasure, x)
    logdensity_def(cm.parent, lazymerge(cm.constraint, x))
end

@inline basemeasure(cm::ConditionalMeasure) = basemeasure(cm.parent) | cm.constraint

This really belongs as part of the interface, so it should be moved to MeasureBase. Also, the constraint could be extended beyond named tuples, so the logdensity_def method should really be

@inline function logdensity_def(cm::ConditionalMeasure{M,C}, x) where {M,C<:NamedTuple}
    logdensity_def(cm.parent, lazymerge(cm.constraint, x))
end