Unify test data creation in /testdata

[achaikou]

At the moment we have at least 3 different approaches on how test files are created:

• separate python script per file (though openvds) in different location than files, no creation instructions
• separate python scrips for main files (per segyio) plus some convert scripts
• many-files in one python script, all created directly on python call

We want to try to unify approaches somewhat, reduce duplication and make it clear and easy how to create each individual test file.

Note that creation data is ingrained into the files, so every re-run would create new file. So it would be good if it was easy to recreate only the file we want to, without touching the other ones. It would also be nice to assure that scripts and created files actually always correspond to each other (same version), but that might be difficult due to date issues.

[achaikou]

vds files created with the same scripts have different dates in two places and two more different lines (which could be some checksum or hash). If we go through segy there are even more differences.

So it seems not feasible to try to assure consistency.