added support for categories of sheaves

- declared IsCategoryOfSheaves
- installed HomalgCategory for [ IsSheafOfRings, IsString ]
- use HomalgCategory in Proj
- installed a view method for IsCategoryOfSheaves
- the functor components
  . ContainerForWeakPointersOnComputedBasicObjects
  . ContainerForWeakPointersOnComputedBasicMorphisms
  are now true or false, where false means
  "don't install for an internal ring"
(needs homalg/Modules/GradedModules v2012.05.17)
renamed HOMALG_SHEAVES_PROJ -> HOMALG_SHEAVES
coding style
*Proj* -> *Scheme*

- added representation IsSchemeRep and derived IsProjSchemeRep from it
- renamed
  . Functor_Hom_for_coherent_sheaves_on_proj -> Functor_Hom_for_coherent_sheaves
  . IsSheafOfRingsOnProjRep -> IsSheafOfRingsOnSchemeRep
  . TheFamilyOfProjSchemes -> TheFamilyOfSchemes
  . TheTypeProjSchemes -> TheTypeProjectiveScheme
  . IsSheafOfRingsOnProjRep -> IsSheafOfRingsOnSchemeRep
- adapted the strings HOMALG_SHEAVES.(short_)description
- added the type TheTypeScheme
fixed a bug in StructureSheafOfProj
commented out obsolete lines Proj
fix conventions
declared the property IsProjectiveSpace

and set it in Proj if known
StructureSheafOfProj now takes care of base rings
started support for affine schemes

- declared the properties
  . IsAffine
  . IsAffineSpace
  for schemes
- declared and installed StructureSheafOfSpec
- declared and installed the affine scheme constructor Spec
- installed a method Scheme for
  [ IsFinitelyPresentedSubmoduleRep and ConstructedAsAnIdeal ]
- enhanced ViewObj to deal with projective and affine schemes
  simultaneously
- installed ViewObj for
  . IsAffineSpace
  . IsProjectiveSpace
- Sheafify is now declared for IsHomalgModule instead of IsHomalgGradedModule
- declared Spec for IsHomalgModule
coding style
started support for divisors

- category: IsDivisor
- representation: IsDivisorRep
- properties:
  . IsPrimeDivisor
  . IsQuasiHomogeneous
  . IsEffective
  . IsBasePointFree
  . IsAmple
  . IsVeryAmple
- attributes:
  . DefiningPolynomial
  . AmbientScheme
  . PrimeDivisorsAttr
  . DerMinusLogMap
  . DerMinusLog
  . DerMinusLogInWeylAlgebra
  . Annihilator1
  . DegreeOfDivisor
- operations:
  . HomalgRing
  . RingOfDerivations
  . DefiningPolynomialOverWeylAlgebra
- constructors:
  . Divisor
  . \+
- view and display methods

- added the component Diff to CommonHomalgTableForLocalizedRingsTools
- use the Diff support for local rings to install a method for
  IsQuasiHomogeneous for local divisors using Saito's theorem
  (Sheaves now depends on LocalizeRingForHomalg v2012.05.19)
fixed a bug in DecideZero (IsZero -> IsArtinian)
adapted CommonHomalgTableForLocalizedRingsTools.Diff

to return a matrix over the local ring complying with
GradedRingForHomalg v2012.05.22
simplified the single argument RingOfDerivations for divisors

using the single argument method RingOfDerivations installed in
MatricesForHomalg/LocalizeRingForHomalg/GradedRingForHomalg v2012.05.25
declared and installed the base change \* for [ IsHomalgRing, IsDivisor ]
renamed IsLocalRing -> IsLocal

complying with MatricesForHomalg 2012.05.27
declared IsLocal for schemes

- set IsLocal( X, IsLocal( R ) ) if HasIsLocal( R ) in StructureSheafOfSpec
- set IsLocal( X, IsLocal( R ) ) if HasIsLocal( R ) in Spec
- enhanced ViewObj for schemes to indicate local (affine) schemes
use Pullback of sheaves in examples

(needs all packages in v2011.10.05)
remove zero rows from the defining matrix of a central arrangement