VOSITables look for <description> tag in child table endpoint

When table descriptions are requested for a VOSITable, the individual table information is fetched from the child table resource, including the `` tag. The `` tag is optional in the [child table resource](http://www.ivoa.net/documents/VOSI/20170524/REC-VOSI-1.1.html#tth_sEc5.4). In at least some cases, the `` tag is present in the tables endpoint but that information is ignored.

We discovered this in the IRSA TAP server with this code:
```
import pyvo
service=pyvo.dal.TAPService('https://irsa.ipac.caltech.edu/TAP')
print(service.tables['akari_irc'].name, service.tables['akari_irc'].description)
```
(warning: the `` tag is likely to be added to the child resources for this service, so this example may start returning descriptions.)

In the `VOSITables._get_table(name)` method, the child resource is queried if `table.columns` and `table.foreignkeys` do not exist. But `table.description` is overwritten with None if the child resource does not include ``.

I propose that if the child resource does not include ``, then the existing table description from the tables endpoint should be used instead. If there is agreement that this is the desired behavior, I can make a pull request to implement this inside `dal.vosi.VOSITables._get_table()`.

Start in dal.vosi.VOSITables._get_table() and trace how the child table resource replaces information from the tables endpoint. Preserve the existing tables-endpoint description when the child resource has no tag, and verify that the example VOSITable description is no longer lost.

Written by the indexing model from the issue text.