Implement data skip in samples

[papadop]

Reference issue (if any)

None

What does this implement/fix?

This implements the handling of the tag FIFF_DATA_SKIP_SAMP.

Additional information

Fiff documentation states:

Data skip is a time segment where no data has been
measured. There are two ways of encoding a data skip. Currently only
only data_skip tag is supported, but any data reading routines should be
designed to handle also data_skip_samples tag. The difference is that
data_skip gives the length of the skip in number of data buffers whereas
data_skip_samples gives the length in samples.

This patch does it.
This is not a feature used that often, but since it is documented that way....