Add options for CMIP6 settings through usermods

[TomasTorsvik]

This PR makes it possible to enable CMIP6 settings for BLOM/iHAMOCC through usermods.

I start including this on the release-1.6 branch, since this is what is being tested currently for NorESM2.3. If this is accepted, I will enable these usermods, with necessary modifications, for the v1.7 and master branches as well.

[matsbn]

As far as I can see, this looks fine!

[jmaerz]

@matsbn and @TomasTorsvik , I wonder, if there is any good reason to put this into usermods - and not into the namelist_definition_blom.xml? - since to my understanding the isopycnic bulkmixlayer setup isn't been further developed, they are currently mostly run in CMIP6-compatible mode anyway, right?

[mvertens]

@TomasTorsvik - I agree with @jmaerz. Here is an alternative proposal:

• A new BLOM xml variable can be defined in config_component.xml - say BLOM_CMIP6
• A new variable can be defined in buildnml - say cmip6_setting
cmip6_setting = case.get_value("BLOM_CMIP6")
• then namelist_definition_blom.xml can use this as follows:

  <entry id="edanis">
    <type>logical</type>
    <category>diffusion</category>
    <group>diffusion</group>
    <values>
      <value>.true.</value>
      <value "cmip6_setting">.false.</value>
    </values>
    <desc>If true, apply anisotropy correction to eddy diffusivity (l)</desc>
  </entry>

The above can modified to also have grid dependent settings.

[jmaerz]

@mvertens , thanks, yes, that would be the other alternative, which could also make sense, if the isopycnic wil be further developed/used differently than with CMIP6 settings.

[mvertens]

I think that an advantage with this approach is the there is only one xml variable that needs to be set - rather than multiple user_mod directories - potentially for different grids.

[TomasTorsvik]

@jmaerz , @mvertens , @matsbn
I can try to implement the namelist_definition_blom.xml, perhaps in a new PR as it is different from the approach in this PR.