Update tutorial/session-4.html#multi-dimensional-maps to WOA18

**Description of the desired feature**

As @PaulWessel pointed out: it's time to update https://docs.generic-mapping-tools.org/6.0/tutorial/session-4.html#multi-dimensional-maps to use data from World Ocean Atlas 2018 (WOA18)

There is no equivalent file in WOA18 (with all four seasons in one) as in the current example of WOA98. There are separate seasonal files, but still with a degenerated (1 value) time dimension.

Summer: https://data.nodc.noaa.gov/thredds/fileServer/ncei/woa/temperature/decav/1.00/woa18_decav_t15_01.nc
Spring:
https://data.nodc.noaa.gov/thredds/fileServer/ncei/woa/temperature/decav/1.00/woa18_decav_t14_01.nc

We could also add to the example a question to use the numerical value of depth, using the parentheses "(" and ")".

**Are you willing to help implement and maintain this feature?**

Yes. I can do the implementation, i.e. changing the text of the example.
Others will need to put the new data in the cache.