Add `complexes` and `r_complex` support

[DavisVaughan]

If I remember correctly, I had this mostly feature complete here:
DavisVaughan#2

I was just waiting on the data_p_ PR, which has been merged

[pachadotdev]

added here #427 (and with matrices too)

[pepijn-devries]

I would like to see this feature in cpp11. Timewise, when could we expect this feature to be merged with the main branch?

[pachadotdev]

I would like to see this feature in cpp11. Timewise, when could we expect this feature to be merged with the main branch?

I don't know, I am just a user doing things for my lab. This is an external contribution that I added now as I found the need to use complex matrices for a problem.

[pepijn-devries]

Thanks for the response @pachadotdev. Maybe someone at @r-lib could have a look at this.

[pachadotdev]

@DavisVaughan Hi! Could you look at this? I kinda need this PR to move forward with some parts of cpp11armadillo

added here #427 (and with matrices too)

[pachadotdev]

@r-lib maybe?

[pachadotdev]

hi @DavisVaughan
could you look at this?