Advanced typemap and full user control

[JonasIsensee]

This PR gives the user full control over type reconstruction.

Simply pass a function (f::JLDFile, typepath::String, params::Vector) -> type as keyword to load or jldopen.
This function is then called for every single type that will be decoded.
The default implementation is JLD2.default_typemap and all standard cases can simply be forwarded to it.

Example:
Here, for bogus reasons we want to load a Complex{Int} as a Complex{Float64} while also modifying the real part during loading.
if typepath == "Base.Complex" && params == [Int] checks that only Complex{Int} get changed and
wrapping the updated type in JLD2.Upgrade ensures that our custom implementation of JLD2.rconvert gets called during loading.

    fn = "advancedtypemap.jld2"
    z = 1 + 2im # Complex{Int}
    zf = 1f0 + 2f0*im # Complex{Float32}
    jldsave(fn; z, type = Complex{Int}, zf)

    typemap = function(f, typepath, params)
        @info "typepath: $typepath, params: $params"
        if typepath == "Base.Complex" && params == [Int]
            return JLD2.Upgrade(Complex{Float64})
        end
        JLD2.default_typemap(f, typepath, params)
    end

    JLD2.rconvert(::Type{Complex{Float64}}, nt::NamedTuple) =
        Complex{Float64}(nt.re + 1, nt.im)

    d = load(fn; typemap)
    @test d["z"] isa Complex{Float64}
    @test d["z"] == 2.0 + 2.0im
    @test d["type"] == Complex{Float64}
    @test d["zf"] isa Complex{Float32}
    @test d["zf"] == 1.0 + 2.0im

closes #630

[codecov]

Codecov Report

Attention: Patch coverage is 98.18182% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.33%. Comparing base (0aae9f4) to head (21f0705).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/data/reconstructing_datatypes.jl	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #650      +/-   ##
==========================================
- Coverage   85.36%   85.33%   -0.04%     
==========================================
  Files          37       37              
  Lines        4407     4404       -3     
==========================================
- Hits         3762     3758       -4     
- Misses        645      646       +1     

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