Use default reader_kw=nothing to make FieldTimeSeries concretely typed

[glwagner]

The default is not a concrete type which may cause performance issues when FieldTimeSeries is used as boundary condition or forcing in a kernel:

Oceananigans.jl/src/OutputReaders/field_time_series.jl

Line 373 in 6c40d7e

reader_kw = Dict{Symbol, Any}())

I suggest nothing as the default. Users can still change it if they want.

We could also use a NamedTuple instead of Dict.

@ali-ramadhan I think you maybe were not thinking that we use FieldTimeSeries in kernels and models now (not just for reading output)

[ali-ramadhan]

Ah good catch! Yeah nothing would be a good default. I can open a PR.

[glwagner]

It could be nice to support both Dict and NamedTuple --- I see uses for both. Dict for post processing and NamedTuple for forcing models. Might already work is reader_kw is splatted in fact.

[ali-ramadhan]

Hmmm, might have to default to NamedTuple as nothing cannot be splatted.

julia> f(x, y; kwargs...) = @show kwargs
f (generic function with 1 method)

julia> N = nothing

julia> f(1, 2; N...)
ERROR: MethodError: no method matching iterate(::Nothing)

Closest candidates are:
  iterate(::Base.AsyncGenerator, ::Base.AsyncGeneratorState)
   @ Base asyncmap.jl:362
  iterate(::Base.AsyncGenerator)
   @ Base asyncmap.jl:362
  iterate(::Core.MethodMatch, ::Int64)
   @ Base deprecated.jl:265
  ...

Stacktrace:
 [1] merge(a::@NamedTuple{}, itr::Nothing)
   @ Base ./namedtuple.jl:361
 [2] top-level scope
   @ REPL[12]:1

[glwagner]

Hmmm, might have to default to NamedTuple as nothing cannot be splatted.

julia> f(x, y; kwargs...) = @show kwargs
f (generic function with 1 method)

julia> N = nothing

julia> f(1, 2; N...)
ERROR: MethodError: no method matching iterate(::Nothing)

Closest candidates are:
  iterate(::Base.AsyncGenerator, ::Base.AsyncGeneratorState)
   @ Base asyncmap.jl:362
  iterate(::Base.AsyncGenerator)
   @ Base asyncmap.jl:362
  iterate(::Core.MethodMatch, ::Int64)
   @ Base deprecated.jl:265
  ...

Stacktrace:
 [1] merge(a::@NamedTuple{}, itr::Nothing)
   @ Base ./namedtuple.jl:361
 [2] top-level scope
   @ REPL[12]:1

That works. I think they are sort of identical in the end