New linear representation type

Many group actions are actually linear representations, which allows to define apply_diff automatically. I would suggest the following:

1. Create a new abstract type AbstractLinearRepresentation:

abstract type AbstractLinearRepresentation{AD <: ActionDirection} <: AbstractGroupAction{AD} end

2. Define apply_diff for this class of group actions:

Manifolds.apply_diff(A::AbstractLinearRepresentation, g, ::Any, X) = apply(A, g, X)

3. Make the existing linear group actions derive from AbstractLinearRepresentation For instance:

struct TranslationAction{TAD<:ActionDirection,TM<:AbstractManifold,TRn<:TranslationGroup} <:
       AbstractLinearRepresentation{TAD}
    manifold::TM
    Rn::TRn
end

3. Clean up existing code Most of the existing implementations of apply_diff follow this pattern (because they are linear representation), for instance:

• apply_diff(::TranslationAction,...)
• apply_diff(::RotationActionOnVector,...)
• apply_diff(::RotationTranslationOnVector,...)`