Simplify the usage of createrepo_c.RepomdRecord

[ghost]

It would save me few letters and one variable name if I could create the createrepo_c.RepomdRecord using a single function :)

E.g. it could be:

• createrepo_c.RepomdRecord(typename, filename, createrepo_c.SHA256) and it will create a "filled" record
• or similarly createrepo_c.RepomdRecord.create_filled(typename, filename, createrepo_c.SHA256)
• or even createrepo_c.RepomdRecord.filled_from_file(xmlfile, createrepo_c.SHA256)
• or even repomd.set_record_auto(typename, filename, createrepo_c.SHA256)
• or even repomd.set_filerecord(xmlfile, createrepo_c.SHA256)

(it overlaps with #19)