Better management of `ln_rnf_icb`

Context

This ticket is a copy/paste of a former open ticket from the NEMO Trac system.

ln_rnf_icb is an option to add a climatology of icb melt. This option is embedded within the rnf module. As it is, using this option in a regional configuration like WED025 (ie no liquid river runoff) is not possible without adding an river runoff file full of 0.

Proposal

To fix the issue mentioned above and ease the setting of icb in the namelist, I suggest to have all the icb related variables inside the namberg namelist bloc with something similar to what is done for the isf with explicit cavities and parametrised cavities :

• a master flag ln_berg to activate the icebergs module
  • ln_icb_clim to trigger the use of an iceberg climatology
  • ln_icb_lagr to trigger the icb lagrangian model.