Add get_ functions

[Robinlovelace]

Building on experience with stats19, get_() is a good function name prefix to clarify that you're about to get some data from the outside world. I suggest the following functions:

• get_centroid() - already there, at least for 2011 Census MSOAs for England and Wales
• get_od() - added in Add get_od() function #13
• get_desire_lines() - added in Add get_od() function #13
• get_pct_zones()
• get_pct_centroids()
• get_pct_lines()
• get_pct_routes_fast()
• get_pct_routes_quiet()
• get_pct_rnet()

Just an idea for now and we don't have to do all of them, but I think it will make it much easier to get the valuable data provided by the PCT.

[layik]

This is what we are talking about! What a feast :)

[layik]

Some of them would need clarifications/assistance for an outsider like me.

[Robinlovelace]

See latest commits: get_pct_zones() is in there. Building on the generic get_pct() function, the others should be quick to add.

[layik]

Wrapper functions, we can document get_pct and it can be used as the main function. If you are happy @Robinlovelace this can be closed.

[layik]

Not quire sure why travis is not happy with installing rgdal.

[Robinlovelace]

We shouldn't need rgdal...

[Robinlovelace]

Awesome work @layik ! Seems done to me so closing.