addDCElement is too slow

[mpadge]

addDCElement is too slow

[eblondel]

@mpadge I've slightly improved the code, first by fetching DC rdf vocabulary by default, (previously done at each DC element creation), secondly by simplifying each DC element class by aligning with the R6 class writing convention used in the package, for what concern native DC elements (for which we don't really need to look into all class inheritance). Together with that, i've provided setters for lists of elements . See #14

[eblondel]

Yes, this is well known issue that I have to look into very soon. I had already improved this in similar package methods (eg geometa) and I have to see if this can be applied here. I didn't know about 'memoise' and I will look into it, thanks for that. Also something you have to know is that the 'add' methods check if there is alread a value, and will return FALSE if already available, but since some elements consist in complex objects (and not simple string values) the comparison is done by comparing the out xml.

An immediate approach, faster, that is actually implemented in the decode function, is not to use the util adders (and then much less convenient), but directly set the fields with the appropriate elements. Example:

dc <- DCEntry$new()
dc[["contributor"]] <- lapply(c("Bob", "Peter", "Bunny"), function(x){DCContributor$new( value = x)})

On this basis, I will support new setters as talked in #14

[mpadge]

Great stuff - thanks!