move hets errors into own module and add new error

Add the new error which is to be raised if a hets-instance is supposed
to be used, but not actually reachable/up.
raise HetsInactiveInstanceError if appropriate.
increase hets-minimal-version and add key

Add key for the general version of hets.  Currently not really important
for version checking purposes, as the timestamp is thorough enough, we
  will need it for better validation.

Necessary changes in hets: wget-headers for ontohub-iri file retrieval.
start extracting uri-get-fetching into class

Since a class should be more useful than a collection
of loosely coupled methods we'll use an instantiable
class for the more complex get-requests.
add method to choose a hets-instance for actions.
provide methods for minimal-version hets-info.
remove obsolete code-document from parsing.
remove xml-data from ontology-version and related

Remove xml_name and pp_xml_name from ontology-version.
This should also include removal of every related method.
adjust parsing to work with io/tempfile instance

In order for the API-system to work properly we'll need
to work with the streamed Tempfile. So instead of
working with filepaths we'll need to pass io-instances
to parsing-processes.