Support string columns

[BradyAJohnston]

Supports string columns, fixes #20

This now additionally imports string columns. It uses numpy.unique(data, return_inverse = True) which alphabetically sorts the strings and assigns them integer values starting at 0.

Strings aren't supported as fields / attributes, but we can use strings inside of the For Each Element zone, turning the field integer value into a single value.

To reliably access this data, a custom node group is created that contains an Index Switch node setup with the string values for that data type.

In the example test data, the resulting nodes would look like below.

Inside the created node group:

[kolibril13]

Just had the chance to do some more testing, and it works great!