RowVector support in v0.7

[schillic]

This is the last missing step toward v0.7 support (#121).

In ExponentialMap.jl we have a function with this signature:

function get_row(spmexp::SparseMatrixExp{N}, i::Int)::RowVector{N} where {N}

We currently use transpose, which in v0.7 results in an object of type LinearAlgebra.Transpose, so we get an error because the type is different from the expected RowVector. In v0.6 the transpose function creates a RowVector.

I cannot find a way to create a RowVector without deprecation warning in v0.7.

To me, the suggested way to transpose a vector is either through transpose (lazy) or permutedims (explicit) (search for "transpose" here). However, the types are different from RowVector:

julia> x = [1., 2., 3.]
3-element Array{Float64,1}:
 1.0
 2.0
 3.0

julia> transpose(x)
1×3 LinearAlgebra.Transpose{Float64,Array{Float64,1}}:
 1.0  2.0  3.0

julia> permutedims(x)
1×3 Array{Float64,2}:
 1.0  2.0  3.0

The above is the only place where we use RowVector, so it might be safe to just change the return type of this function. Should we do that? To which type?
Note that we need a solution that also works in v0.6, but we can always write a helper function that does the right thing.

We can also create a Vector out of it:

julia> @time transpose(x)[:]
  0.000025 seconds (6 allocations: 288 bytes)
3-element Array{Float64,1}:
 1.0
 2.0
 3.0

# faster but one more allocation
julia> @time permutedims(x)[:]
  0.000012 seconds (7 allocations: 368 bytes)
3-element Array{Float64,1}:
 1.0
 2.0
 3.0

[mforets]

I would suggest to remove the output type annotation altogether. In v0.7 you will get a LinearAlgebra.Transpose{Float64,Array{Float64,1}} while in v0.6 a RowVector{Float64,Array{Float64,1}}.

[mforets]

PS: there is no performance impact on this change, and the function is type stable after inspecting with @code_warntype.

[schillic]

Okay, since there seems to be no good solution, I agree. We should add a note in the comment, though, and maybe add a unit test to check that type in the respective version.

[mforets]

👍 for adding a comment in the docstring