geo.ttl should not use the empty prefix

[VladimirAlexiev]

geo.ttl:

PREFIX : <http://www.opengis.net/ont/geosparql#>

However, the spec and other turtle files use the prefix geo: for this.

Using an empty prefix is IMHO a bad practice (started as early as SKOS) since:

• It sets a bad example: there's only one empty prefix, so clients cannot use empty prefix in their own data
• Semantic repositories often use prefixes defined in ingested files as "namespaces" to be used in prefix autocompletion in SPARQL. But you would not want GeoSPARQL to be associated with :
• It makes search in the ontology file harder. In fact this issue was mis-described as "geo:SpatialObject is not defined in geo.ttl or any of the other ontologies" because of this problem.

In addition, GeoSPARQL should declare its namespace using VANN:

<http://www.opengis.net/ont/geosparql> 
  a owl:Ontology ;
  vann:preferredNamespacePrefix  "geo" ;
  vann:preferredNamespaceUri     "http://www.opengis.net/ont/geosparql#" ;