support for libsing via external GAP
use more installed GAP operations in libsing instead of SI_*
now HomalgFieldOfRationalsInLibSing is the default
spacings
declared and installed SIH_Get*UnitPositions

+ added corresponding entries to
  . CommonHomalgTableForLibSingTools
  . CommonHomalgTableForExternalLibSingTools
call SI_LIB( "matrix.lib" ); and use SIL_dsum and SIL_tensor for the moment