Feature/add condition number calculation

• Add a routine to calculate the l2 norm of complex 2d arrays

• Add calculation and printing of the response matrix condition number for fields local (only if local debug mode enabled for now)

• Minor comment

• Add note about work duplication

L2 norm calculation may belong in utils but I’ve added it directly to fields_local here as this is the only place it’s currently required and we’d probably prefer to use the language standard norm2 for real data but this adds more complication than required currently (only supports reals, only in f2008 etc.).