Support string XDF streams

[cbrnr]

Currently, strings streams are only supported if they have a sampling frequency of zero (i.e. marker streams). However, regularly sampled streams (i.e. fs > 0) can also have a dtype of string, which is currently not implemented. I'm not sure how to best handle such string streams, converting to annotations seems like the best idea (but could be very inefficient if the sampling frequency is high).