Add some methods to make JLSOFile more Dict-like

[nickrobinson251]

• adds keys, haskey, get and get! for JLSOFile (but not iterate as i didn't have a use-cae for it); closes #107
• fixes JLSOFile() to return a JLSOFile, whereas on master it errrors:

  julia> JLSOFile()
  ERROR: MethodError: no method matching JLSOFile(::Dict{Union{}, Union{}}; version=v"4.0.0", julia=v"1.6.0", format=:julia_serialize, compression=:gzip, image="")
  Closest candidates are:
  JLSOFile(::Any, ::Any, ::Any, ::Any, ::Any, ::Any, ::Any, ::Any, ::Any) at /Users/nick/repos/JLSO.jl/src/JLSOFile.jl:2 got unsupported keyword arguments "version", "julia", "format", "compression", "image"
  JLSOFile(; version, julia, format, compression, image, kwargs...) at /Users/nick/repos/JLSO.jl/src/JLSOFile.jl:70
  JLSOFile(::Pair{Symbol, B} where B...; kwargs...) at /Users/nick/repos/JLSO.jl/src/JLSOFile.jl:89
  ...
  Stacktrace:
  [1] JLSOFile(; version::VersionNumber, julia::VersionNumber, format::Symbol, compression::Symbol, image::String, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ JLSO ~/repos/JLSO.jl/src/JLSOFile.jl:78
  [2] JLSOFile()
   @ JLSO ~/repos/JLSO.jl/src/JLSOFile.jl:78
  [3] top-level scope
   @ REPL[2]:1

• adds a save(io, ::JLSOFile) method

Together, this allows users to write code like

jlso = isfile(filename) ? read(filename, JLSOFile) : JLSOFile()
get!(jlso, :qux) do
    make_qux()
end
JLSO.save(filename, jlso)

or

if isfile(filename) 
    jlso = read(filename, JLSOFile)
    haskey(jlso, :qux) && return jlso[:qux]
else
    jlso = JLSOFile()    
end
jlso[:qux] = make_qux()
JLSO.save(filename, jlso)

The main advantage is this allows working with a JLSOFile similar to working with the Dict that would be returned by JLSO.load(filename), but with JLSOFile lazily deserialising (unlike load)

[codecov[bot]]

Codecov Report

Merging #108 (0a08047) into master (dbf7be6) will increase coverage by 0.11%. The diff coverage is 100.00%.

:exclamation: Current head 0a08047 differs from pull request most recent head 7c75b90. Consider uploading reports for the commit 7c75b90 to get more accurate results

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   97.56%   97.67%   +0.11%     
==========================================
  Files           6        6              
  Lines         205      215      +10     
==========================================
+ Hits          200      210      +10     
  Misses          5        5              

Impacted Files	Coverage Δ
src/JLSOFile.jl	93.33% <100.00%> (+2.85%)	:arrow_up:
src/file_io.jl	94.44% <100.00%> (+0.10%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update dbf7be6...7c75b90. Read the comment docs.