Define Base.axes1(S::Base.Slice{<:BlockedOneTo})

Define `Base.axes1(S::Base.Slice{<:BlockedOneTo}) = S.indices`, analogous to the definition of [`Base.axes(S::Base.Slice{<:BlockedOneTo})`](https://github.com/JuliaArrays/BlockArrays.jl/blob/90ddfa96fb0ade3da4f67ac285fcb340350c19c6/src/blockaxis.jl#L680) and [`Base.axes1(S::Slice{<:OneTo})`](https://github.com/JuliaLang/julia/blob/cd446800a189704062bbd58970f1cff4fbc52fc1/base/indices.jl#L391).

Closes #484.

Author: mtfishman

src/blockaxis.jl

@@ -678,6 +678,7 @@ Base.summary(io::IO, a::AbstractBlockedUnitRange) =  _block_summary(io, a)
 ###
 
 Base.axes(S::Base.Slice{<:BlockedOneTo}) = (S.indices,)
+Base.axes1(S::Base.Slice{<:BlockedOneTo}) = S.indices
 Base.unsafe_indices(S::Base.Slice{<:BlockedOneTo}) = (S.indices,)
 blockaxes(S::Base.Slice) = blockaxes(S.indices)
 @propagate_inbounds getindex(S::Base.Slice, b::Block{1}) = S.indices[b]