Small performance improvements

[juliasloan25]

Minor issues found in #390 that we want to clean up for better performance.

Tasks

• don't pass FT as an argument
• use zero(FT) instead of FT(0) to reduce allocations use zero(FT) instead of zero(field) in broadcasts #1599
• remove allocations in combine_surfaces #1595
• store fields as NamedTuple of Fields #1768
• remove use of parent (except where needed when spaces aren't identical ?)
• make sure no closures over global variables (e.g. FT
• limit use of Val to only when necessary

some points taken from #323