fix extract_message: use typing.List instead if list (for older pytho…

…n versions)

README: update link to docs

core: add header-only log file load capability

ulog2csv: handle '/' in data name

db: add database migration tool

Database setup required running ".read sql/setup_db.sql" manually, which
was unintuitive when using pyulog as a library. This commit adds the
ulog_migratedb console script to handle the database setup.

A single SQL script isn't robust to future database changes. If there is
a discovered issue with the current database schema, or there is a bug
in the schema, then new changes might be needed, and we really don't
want to lose data by accident. Hence, careful data migration might be
needed.

Other migration tools like "alembic" and "migrations" were considered,
but the former was sqlalchemy-specific and the latter seemed to have
little support. Hence a small migration script was created,
migrate_db.py, which handles simple forward-migration. This might be
overengineered, but the migration logic itself is only 50 lines or so,
so it might be worth the future-proofing.

versioneer: update to 0.19

robustify against log files with corrupt messages

Merge pull request #44 from devbharat/feature/add_sync_recovery

Added sync msg recovery

Merge branch 'master' of github.com:PX4/pyulog

ulog2kml: use vehicle_status instead of commander_state for the fligh…

…t mode