View int8 and uin8 Data as binary string or encoded string

In python strings may be stored as encoded bytearray which in contrast to `\0` terminated byte stringscan be compressed. The resulting 8 bit integer dataset is displayed as such by hdf-compas. Which makes inspections of such datasets a bit difficult. Therefore i do suggests to enhace dataset view for 8bit integer type datasets by providing alternative views like the following crude examples:

```
binary ....... view as plain binary bytes eg. print(bytearray(eight_bit_int_dataset[()]))

hex .......... view as hexstring instead eg. print("{:x}".format(eight_bit_int_dataset[()]))

... view as decoded bytarray content eg.
selected_encoding="utf8"
print(bytearray(eight_bit_int_dataset[()]).decode(selected_encoding))
```
That would allow to inspect hdf5 which encodes strings in eight bit int datasets instead of `\0` terminated c-style string datasets.

**NOTE** as python2 is out of support this feature request shall only affect python3 branch #186