Add capability for crawlers RetrieveAVRISNGData.py to write .met files

[lewismc]

It would be helpful for us to capture metadata pertaining to the source of .img and .hdr files we download from the AVIRIS FTP server.
For example, if we wrote an accompanying .met file for each file we download via RetrieveAVRISNGData.py, we could use the .met files to augment the metadata with the Dublin Core dcterms:source metadata key which would enable anyone to obtain the original data should they wish. Further description of what dcterms:source is can be found below.

dcterms:source
    dcterms:description "The described resource may be derived from the related resource in whole or in part. Recommended best practice is to identify the related resource by means of a string conforming to a formal identification system."@en ;
    dcterms:hasVersion <http://dublincore.org/usage/terms/history/#sourceT-001> ;
    dcterms:issued "2008-01-14"^^<http://www.w3.org/2001/XMLSchema#date> ;
    dcterms:modified "2008-01-14"^^<http://www.w3.org/2001/XMLSchema#date> ;
    a rdf:Property ;
    rdfs:comment "A related resource from which the described resource is derived."@en ;
    rdfs:isDefinedBy <http://purl.org/dc/terms/> ;
    rdfs:label "Source"@en ;
    rdfs:subPropertyOf <http://purl.org/dc/elements/1.1/source>, dcterms:relation ;
    skos:note "This term is intended to be used with non-literal values as defined in the DCMI Abstract Model (http://dublincore.org/documents/abstract-model/).  As of December 2007, the DCMI Usage Board is seeking a way to express this intention with a formal range declaration."@en .