Polish README

Skip example of `compute_sea_state_1d_spectrum()` on CRAN only

Ready for 0.5.1

Merge branch 'main' of https://github.com/Resourcecode-project/r-reso…

…urcecode

Change Examples in `compute_sea_state_2d_spectrum()` to be run only w…

…hen NOT on CRAN

Increment version number to 0.3.0

Fix code style