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.
It does it by adding the handling of the tag in the reading loop.
Also, the implementation of the initial data skip is changed by leveraging the in-loop reading code instead of duplicating it as it was. In addition, the new code should handle correctly multiple FIFF_DATA_SKIP_SAMP or FIFF_DATA_SKIP, which was not the case previsouly.

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....

[larsoner]

Do you have or could you create some tiny test file that has this attribute that we could add to mne-testing-data?

[papadop]

I will do that. Would a variant of intervalrecording_raw.fif be OK (at 12MB not sure it can be considered as tiny) ? If it is, I already have it, it will just remain to write the test down. Otherwise, I need to create a smaller one which would require a bit more time (but normally it shouldn't be too difficult).

[larsoner]

Ideally < 1 MB would be better so if you can shrink it please do!

[papadop]

OK. Will do.