Function handle support

[matthijscox]

We don't error anymore on function handles since MAT v0.11, but let's remember to provide good read-write support at some point:

Current behavior:

julia> sin_f = matread("test/v7.3/function_handles.mat")["sin"]
Dict{String, Any} with 4 entries:
  "function_handle" => Dict{String, Any}("function"=>"sin", "file"=>"", "type"=>"simple")
  "sentinel"        => '@'
  "separator"       => '/'
  "matlabroot"      => "/opt/MATLAB/R2018b"

julia> anon = matread("test/v7.3/function_handles.mat")["anonymous"]
Dict{String, Any} with 4 entries:
  "function_handle" => Dict{String, Any}("function"=>"sf%0@(x)x", "workspace"=>MatlabOpaque("any"=>Any[MatlabOpaque(); Dict{String, Any}();;]), "file"=>"", "within_file_path"=>"__base_function", "ty…
  "sentinel"        => '@'
  "separator"       => '/'
  "matlabroot"      => "/opt/MATLAB/R2018b"

When we write these, you get a struct in MATLAB.

[foreverallama]

I think we can already load function handles already, but as a struct/dict. We could use another wrapper MatlabFunction for this, it's what SciPy (and matio) uses.

I can take this up along with write support.

[foreverallama]

Can you add the type for MatlabFunction or something like that? The data would be a 1x1 struct in MATLAB or a Dict in Julia. I'm not very familiar with the syntax and don't want to spend too much time on it.

I'll update the read and write methods accordingly

[matthijscox]

I see. Well we can go easy-mode and make another dict wrapper, like MatlabOpaque:

struct FunctionHandle
    d::Dict{String,Any} # or OrderedDict
end
# add dict-like interface methods for ease of use? 
# do we even need them? or we just write `FunctionHandle.d` as a struct with some extra HDF5 wrapping?

But if the function handle always has these 4 fields, it might be better for code readability to create a special type. Though we'd have to do more research to figure out what the options are per property...

struct FunctionHandle
    function_handle::Dict{String, Any} # or OrderedDict
    sentinel::Char
    separator::Char
    matlabroot::String
end

Is the order of the fields import by the way, for writing? I guess we'd have to find out.

[foreverallama]

Lets keep it easy. Function handle serialization gets weird when they're nested or a handle to a classdef method