New in-place functions

[LuEdRaMo]

I noticed there is no method of TaylorSeries.div! to divide a TaylorN by a NumberNotScalar, which would be an equivalent of the following:

TaylorSeries.jl/src/arithmetic.jl

Lines 1049 to 1053 in c2698db

	function div!(v::Taylor1{T}, a::Taylor1{T}, b::NumberNotSeries,
	k::Int) where {T<:Number}
	@inbounds v[k] = a[k] / b
	return nothing
	end

but for a TaylorN.

In addition, as discussed online, it would be helpful to have an in-place function to update only the constant term of an AbstractSeries, something like:

constant_term!(c::Taylor1{T}, a::T) where {T <: NumberNotSeries} = c[0] = a

with analogous methods for Taylor1{Number}, TaylorN, HomogeneousPolynomial, etc.