The getter used some regular expression based on the string of the 'verb'
parameter. The 'verb' in question here is 'ListRecords' and it also starts the
initiation of a data block. However, xml allows explicit name spaces and thus
this query fails in case we get qualified xml.

According to the specs of OAI-PMH the server should return a 'noRecordsMatch'
error. (Effectively, being the reason for the missing ListRecords stanza.). Thus
check for the existence of this error instead gives a valid check.

Additionally, the "parsing" fo the resumptionToken also assumed that
we do never get any namespace. This is hotfixed by a more general
regexp, that allows for the inclusion of namespaces, but ultimately
one should consider real XML parsing. (cf. FIXME)

Introduced to cope with OpenAIRE project set.
remove_duplicates() relied on regexping the results to retrieve duplicate
oai-keys from a source and remove the duplicate records. This, however, does not
work with qualified xml containing name spaces. The old method will just corrput
the XML output.

Thus introduce xml parsing by means of lxml to remove the dupes.