BLOM v1.7.1 tag

[TomasTorsvik]

To be concluded in time for noresm3_0_alpha01

• Re-introduce the Externals_BLOM.cfg file for building with MCT in NorESM2.3.X (Add Externals_BLOM.cfg file for use with NorESM2.3 #471)
• Filtering of squared buoyancy frequency and Richardson number for reduced noise in vertical diffusivity (Filtering of squared buoyancy frequency and Richardson number #480).
• Parameterized wave forcing of Langmuir mixing (Enabled parameterized wave forcing of Langmuir mixing. #481).
• Include fix for negative salinity values (Ensure non-negative tracer values and conservation by virtual flux correction #485)