make test data smaller

[gdementen]

For a reason which escapes me, population_session.h5 is 3Mb (while the same data in .xlsx is 11.8Kb) and demography.* are a bit large too, making the total size of the larray package larger than I would prefer.

[alixdamman]

In the PR #771 I have started to replace the use of demography dataset by the demography_eurostat dataset (which is actually population_session renamed).

Is demography used in the unittests?

[gdementen]

I don't remember...

[alixdamman]

I temporarily changed the name of demography, ran the tests and found that demography is only used in the load_example_data() function and in the tutorial.

However, there is still a weird problem with demography_eurostat.h5. I'll take a look.

[alixdamman]

After some tests I discovered that the size of demography_eurostat.h5 is huge because the session contains Axis objects with labels of dtype '<U'.
There is clearly a problem when dumping axes with unicode labels to an HDF5 file.

[gdementen]

can we solve this?

[alixdamman]

We need to convert unicode labels to bytes