* arts-2-1-845

* src/file.{cc,h} (find_xml_file):

Added. This method also outputs a warning to out1 if a .xml and
.xml.gz version of the input file exists. (Suggested by Richard)

* src/xml_io.cc (xml_read_from_file,
xml_read_arts_catalogue_from_file):

Use new find_xml_file function to avoid code duplication.

* src/m_abs.cc (abs_cont_descriptionInit):

Prepend abs_cont_ to variable names to avoid name clash with global
parameters variable (clang 5.1).

* src/methods.cc: Add info about extensions automatically being added
to filenames by ReadXML. (Suggested by Manfred)

git-svn-id: https://www.sat.ltu.se/svn/rt/arts/trunk@8809 aaf1aab0-4228-0410-ad68-8dceda47f409