API: populate root namespace

[neutrinoceros]

I would like that import nonos be the only import statement one needs to leverage the library.
This would improve discoverability.

I also want it to provide high(er) level APIs that would make GasDataSet redundant.

For instance

import nonos

db = nonos.Database(directory=...)
...

time_series = (
    db
    .select_range(15, 25)
    .azimuthal_average()
)
for ds in time_series:
    ...