function resolve( value ) {
	var t = ( typeof value );
	if ( t === 'string' ) {
		return ( str2enum( value ) === null ) ? null : value;
	}
	if ( t === 'number' ) {
		return enum2str( value );
	}
	return null;
}
Returns the diagonal type string associated with a supported BLAS diagonal type value.

@param {*} value - diagonal type value
@returns {(string|null)} diagonal type string or null

@example
var str2enum = require( '@stdlib/blas/base/diagonal-type-str2enum' );

var v = resolve( str2enum( 'unit' ) );
// returns 'unit'