core.R: Add support for PVector

[gaocegege]

Close #241

I am going to support more user cases about statistical computing, PVector is the basis.

Signed-off-by: Ce Gao ce.gao@outlook.com

[jeremydouglass]

Interesting!

Table might be good to import from the core as well for working with large sets of 2D or 3D coordinates -- loadTable and saveTable are already present, but they wouldn't work without Table. At least, I think it is not already included, because it is not listed in the reference....

• https://processing-r.github.io/reference/loadTable.html
• https://processing-r.github.io/reference/Table.html

[gaocegege]

OK，I will take a look