Update default data sources

[raymondben]

Consider updating the default data set choices. e.g. readtopo() defaults to "gebco_08", which is dated now to say the least. Users might well be unaware that this is what they are being given. It would probably be better to keep the default here to whatever the most recent GEBCO product is.
Other reader functions might be in a similar situation.

[mdsumner]

yep good one, readtopo fixed: 14e2f01

[Maschette]

similar but different, the kerguelen dataset listed:

Kerguelen Plateau Bathymetric Grid, https://data.gov.au/dataset/ds-ga-a05f7893-007f-7506-e044-00144fdd4fa6/details?q=

has been superseded by this one:
https://ecat.ga.gov.au/geonetwork/srv/eng/catalog.search#/metadata/147703

if possible could we get it updated?