Refactor hardcoded GMT defaults

[molinav]

This pull request adds the default values for GMT 5.4.5 as they appear in a self-compiled GMT 5.4.5 under Ubuntu 20.04. At least the TIME_LANGUAGE parameter has been renamed as GMT_LANGUAGE, and this is causing warnings to stdout (see issue #7) if GMT 5.4.5 is used with the defaults for 5.1.2 (which are the last ones available in gmtpy).

[molinav]

I changed the default page orientation from landscape to portrait because I saw this is the default value for older versions and otherwise the examples are generated with a wrong layout.

[molinav]

I have added some additional changes and therefore renamed the pull request:

1. I have aligned all the "parameter = value" rows as in the defaults from the GMT 5 series.
2. It is not necessary to keep track of GMT defaults for patch versions if the defaults of the previous GMT are identical, since gmtpy will pick those as defaults if it cannot find for the specific GMT versions. For example, I observed that the defaults for GMT 4.3.0 and GMT 4.3.1 are completely identical. It is enough to have the ones from 4.3.0 and if somebody uses 4.3.1 gmtpy will already take the defaults of GMT 4.3.0 as fallback.
3. After making this change, I realised that it would be nice to just keep track of the defaults by minor version, e.g. the GMT 5.4.z series should not change the defaults between patch versions and so on. For this reason, I renamed the keys in the defaults dictionary so that the entries are always in the form X.Y.0.

In particular, point 3 is important because this pull request was initially adding just defaults for GMT 5.4.5. Later, I was testing in another machine which happened to have GMT 5.4.4, and therefore the defaults were redirected to the previous defaults in gmtpy, which were from GMT 5.1.2 and not compatible. Just adding my original defaults as being for GMT 5.4.0 and not 5.4.5 solved the problem.