Generate dataframe types with added/removed columns

# Extend

To extend a datafram we should be able to do:
``` c++
candas::ihdf df = dataframe{};

/* extend last */
auto df2 = df.extend_last(std::vector(df.size())); // decltype(df2) should be
// optional: values from argument is moved into the new column
// OR
auto df2 = candas::extend_last(std::move(df), std::vector{});

/* extend first */
auto df3 = df.extend_first(std::vector(df.size())); // decltype(df3) should be
// optional: values from argument is moved into the new column
// OR
auto df3 = candas::extend_first(std::move(df), std::vector{});
```

-----

# Remove

To remove a column we should be able to do:
``` c++
candas::ihdf df = dataframe{};

std::vector vec{};

auto df2 = df.remove_col<1>(*vec); // decltype(df2) should be
// optional: removed column is moved into argument vec
// OR
auto df2 = candas::remove_col<2>(std::move(df), *vec);
```