Support for iterating over a text column (supercedes #34)

[deads]

ParaText::CSV::ColBasedLoader loader;
loader.force_semantics("Name", ParaText::Semantics::TEXT);
ParaText::ParseParams params;
params.num_threads=1;
loader.load("earthquakes.csv", params);
for (auto it = loader.column_begin<std::string, false>(0); it != loader.column_end<std::string, false>(0); it++) {
  std::cout << *it << std::endl;
}