More types and fields can be mapped to references in RIS format

There is number of types and fields in RIS format which are mapped to nowhere by default, but there are appropriate reference types and fields where they actually can be mapped.

Examples of fields: AN (Accession Number), CN (Call Number), DO (DOI), PP (Place Published), RP (Reprint Edition).
Examples of types: AGGR (Aggregated Database), CHART (Chart), CPAPER (Conference Paper), EBOOK (Electronic Book), LEGAL (Legal Rule).

There is also number of types which should be mapped to different types than now. For example, SLIDE, STAT, etc.