IsHomalgRing now lies in the filter IsRingWithOne

comply with MatricesForHomalg v2013.04.15
renamed BettiDiagram -> BettiTable

complying with homalg & GradedModules v2013.06.03
updated syntax for Annihilator
adapted bounds to visualize the Serre vector bundle criterion in HorrocksMumford.g
installed immediate method: IsEmpty <=> Dimension( X ) < 0
declared scheme property IsQuasiAffine

(needed for new package AlgebraicThomas)
declared the attribute Closure for (embedded) schemes

(needed for new package AlgebraicThomas)
activated LogicalImplicationsForSchemes

. IsAffine => IsQuasiAffine
. IsEmpty => IsAffine

(needed for new package AlgebraicThomas)
simplified view method for schemes

depends on the activated LogicalImplicationsForSchemes
fixed view method for schemes
view method for empty sheaves now shows the base ring
declared several operations for (embedded) schemes

. Union2 (use it via Union)
. Intersect2 (use it via Intersect)
. Complement
. Difference
. IsSubset

(needed for new package AlgebraicThomas)
declared several properties for schemes/subschemes, subsets

. IsAffineSubscheme
. IsQuasiAffineSubscheme
. IsQuasiProjective
. IsConstructibleSet
a hack for Thomas Bächler to support affine schemes

the package has to be completely redesigned and rewritten from scratch
Schemes: Add IsAffineSchemeRep representation for objects created by Spec.
Declare IsConstructibleSubsetOfAffineSpace property.
Add some obvious logical implications for schemes.
Set IsConstructibleSubsetOfAffineSpace and IsAffineSubscheme in Spec( ).

Also adjust the condition in the ViewObj to use IsConstructibleSubsetOfAffineSpace
instead of IsAffine to decide whether our object is a subobject of affine n-space.
Don't fail in ViewObj if an object has IsAffineSpace, but has no StructureSheaf set.
Declare ComplementAttr as an attribute.

We would like to declare Complement as an attribute,
but the CRISP package uses it as an operation.