audb.load_to() use cache for tables

[hagenw]

Closes #220

This loads missing tables from the cache to the build folder.
In addition, we avoid storing the tables an additional time to the db_root folder.

It stores the tables in the cache under the version number as given by the dependency table,
this avoids caching the same table again for each version, but this is not something that is already integrated in audb.load_table().

Execution time

I measured execution time by running

audb.load_to(build_dir, name, version=previous_version, only_metadata=True, num_workers=8)

for a large database (salamander-a) with lots of tables.

Branch	Cached	Time
master	no	485.0 s
master	yes	485.0 s
this	no	450.0 s
this	yes	4.5 s

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What is not ideal yet, is that by using audb.load_table() to load single tables we block the processes when several tables need to be stored in the same cache folder.

So maybe we should target a general refactoring of the table storing code.

[codecov]

Codecov Report

Merging #221 (3d5de1e) into master (b9bd498) will not change coverage.
The diff coverage is 100.0%.

Impacted Files	Coverage Δ
audb/core/info.py	100.0% <ø> (ø)
audb/core/load_to.py	100.0% <100.0%> (ø)

[hagenw]

Another workaround would be to recommend using the same ../build folder when publishing a new version, but make sure to delete all media files inside the build folder first (which is of cause not a nice solution).