Changes to better implement textConnections

[stranda]

I have added two tests associated with textConnections. I also constructed an "example section" for read_populations to show how textConnections could be used.

While constructing them, I noticed that missing data in a text connection caused an issue associated with data frame column types (converted int to chr). .read_columns() was changed to convert these columns back to int if a textConnection is involved and if they are not already integer

In the end three files have been changed: read_population.R, read_population.Rd (created by running devtools::document()), and test-read_population.R