divrem with flag complete_reduction

[hannes14]

to use for reduce/redeuce_with_quotient/reduce_with_quotient_and_unit
@ederc,@wdecker

[ederc]

There are two comments I want to make:

1. I am not sure if divrem really is doing what we want. Looking at the following example I expect that the last two commands should return the same matrix:

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"])
(Singular Polynomial Ring (QQ),(x,y),(dp(2),C), spoly{n_Q}[x, y])

julia> a = Ideal(R,[x^2+x*y])
Singular ideal over Singular Polynomial Ring (QQ),(x,y),(dp(2),C) with generators (x^2 + x*y)

julia> b = Ideal(R,[x+y])
Singular ideal over Singular Polynomial Ring (QQ),(x,y),(dp(2),C) with generators (x + y)

julia> quo, rem, unit = divrem(a,b);

julia> Matrix(a)*Matrix(unit)
[x^2 + x*y]

julia> Matrix(b)*Matrix(quo)+Matrix(rem)
[x^2 + 2*x*y + y^2]

Or to be more explicit: I do not know where the +y is coming from looking at quo:

julia> Matrix(quo)
[x + y]

2. Don't we need the complete_reduction flag also for reduce in order to be consistent?

[hannes14]

reduce: see #682