Digital reader/writer classes don't support multi-sample line reads/writes

[bkeryan]

The digital line read/write methods in DigitalSingleChannelReader, DigitalMultiChannelReader, DigitalSingleChannelWriter, and DigitalMultiChannelWriter only support single-sample reads/writes, not multi-sample reads/writes.

Note: The underlying C API functions, DAQmxReadDigitalLines and DAQmxWriteDigitalLines, support multi-channel, multi-sample, multi-line reads and writes, where the data is basically a 3D array of (channels, samples, lines) or (samples, channels, lines) depending on fill mode.

Workaround: use the simplified task.read() and task.write() methods.

When this is fixed, we should update the internal test_cached_samples.py test.