Override Base.sum!

[LuEdRaMo]

This PR leverages TS.add! to reduce allocations when summing an array of Taylors. For instance:

# Generate a vector of random Taylor1s
order = 25
n = 10_000
v = [Taylor1(randn(order)) for _ in 1:n]
# New sum! method
y = zero(v[1])
sum!(y, v)
# Compare with existing methods
y == +(v...) # true
y ≈ sum(v) # true
# Benchmark
@time +(v...)
# 0.002061 seconds (30.00 k allocations: 3.357 MiB)
@time sum(v)
# 0.001163 seconds (10.00 k allocations: 2.441 MiB)
@time begin
    y = zero(v[1])
    sum!(y, v)
end
# 0.000688 seconds (3 allocations: 320 bytes)

[lbenet]

Thanks a lot for this addition! I just restarted the problematic job (in the tests); I guess it will run ok. Please ping me when you want me to review this PR.

[lbenet]

Outside from this for-loop, I suggest to add a method for sum! with the signaturesum!(v::TaylorN{S}, a::AbstractArray{HomogeneousPolynomial{S}}), which is used in some evaluate and/or related functions.

[LuEdRaMo]

Thank you for the suggestion! I've added the corresponding method, although I don't know which method of evaluate will benefit the most from it.

[lbenet]

Thanks a lot @LuEdRaMo. I'll go ahead and merge it; in a separate PR, I shall use sum! to optimise some parts of the code.