Saving a JLSO file that is already constructed seems weird

[oxinabox]

I suspect this is because one is suppose to use write not save if it is already constructed?
We should have some docs that clarify

• save/load
• write/read
• setindex/getindex

Demo of it ibeing weird:

julia> breakfast_jlso = JLSOFile("food"=>"☕️🥓🍳", "cost"=>11.95, "time"=>Time(9,0))
JLSOFile([cost, time, food]; version=v"2.0.0", julia=v"1.2.1-pre.0", format=:julia_serialize, compression=:gzip, image="")

julia> JLSO.save("breakfast.jlso", breakfast_jlso)

julia> JLSO.load("breakfast.jlso", "cost")
[warn | JLSO]: KeyError: key "cost" not found
ERROR: KeyError: key "cost" not found
Stacktrace:
 [1] getindex at ./dict.jl:477 [inlined]
 [2] getindex(::JLSOFile, ::String) at /Users/oxinabox/.julia/packages/JLSO/1C0be/src/serialization.jl:57
 [3] load(::IOStream, ::String) at /Users/oxinabox/.julia/packages/JLSO/1C0be/src/file_io.jl:78
 [4] #9 at /Users/oxinabox/.julia/packages/JLSO/1C0be/src/file_io.jl:70 [inlined]
 [5] #open#312(::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::typeof(open), ::getfield(JLSO, Symbol("##9#10")){Tuple{String}}, ::String) at ./iostream.jl:375
 [6] open at ./iostream.jl:373 [inlined]
 [7] load(::String, ::String) at /Users/oxinabox/.julia/packages/JLSO/1C0be/src/file_io.jl:70
 [8] top-level scope at REPL[23]:1
caused by [exception 1]
KeyError: key "cost" not found
Stacktrace:
 [1] getindex at ./dict.jl:477 [inlined]
 [2] getindex(::JLSOFile, ::String) at /Users/oxinabox/.julia/packages/JLSO/1C0be/src/serialization.jl:52
 [3] load(::IOStream, ::String) at /Users/oxinabox/.julia/packages/JLSO/1C0be/src/file_io.jl:78
 [4] #9 at /Users/oxinabox/.julia/packages/JLSO/1C0be/src/file_io.jl:70 [inlined]
 [5] #open#312(::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::typeof(open), ::getfield(JLSO, Symbol("##9#10")){Tuple{String}}, ::String) at ./iostream.jl:375
 [6] open at ./iostream.jl:373 [inlined]
 [7] load(::String, ::String) at /Users/oxinabox/.julia/packages/JLSO/1C0be/src/file_io.jl:70
 [8] top-level scope at REPL[23]:1

julia> JLSO.load("breakfast.jlso", "data")
Dict{String,Any} with 1 entry:
  "data" => JLSOFile([cost, time, food]; version=v"2.0.0", julia=v"1.2.1-pre.0", format=:julia_serialize, compression=:gzip, ima…

[rofinn]

Yeah, I think read and write were supported first. save and load were added later to align with the FileIO ecosystem, but we should probably restrict that to something like

JLSO.save("breakfast.jlso", name1 => obj1, name2 => obj2, ...)

Seems like that would also fit the proposed load API better.