Use Compat library to help with forward looking language

[tlienart]

Language breaking changes: https://github.com/JuliaLang/julia/blob/master/NEWS.md

A few things have already been sorted like abstract Blah to @compat abstract type Blah end etc. A few more things need to be fixed in order for PDMP to be callable by 0.6 and 0.7.

This issue to list what's currently not sorted. (i.e. when include("PDMP.jl") in more advanced Julia fails)

Current error to investigate (with 0.6)

ERROR: LoadError: LoadError: ArgumentError: invalid type for argument pq in method definition for ls_firstbranch! at /Users/tlienart/.julia/v0.5/PDMP/src/local/simulate.jl:157
Stacktrace:
 [1] include_from_node1(::String) at ./loading.jl:539
 [2] include(::String) at ./sysimg.jl:14
 [3] include_from_node1(::String) at ./loading.jl:539
 [4] include(::String) at ./sysimg.jl:14
while loading /Users/tlienart/.julia/v0.5/PDMP/src/local/simulate.jl, in expression starting on line 517
while loading /Users/tlienart/.julia/v0.5/PDMP/src/PDMP.jl, in expression starting on line 46

[tlienart]

So this runs perfectly well with Julia 0.5* and for some reason 0.6 does not like it

[tlienart]

Ok so the inclusion of every file in PDMP.jl works apart from the very last line include("local/simulate.jl") due to problem mentioned above.

[tlienart]

Ok so it seems the problem was that PriorityQueue has been moved out of Base.Collections to DataStructures as per JuliaLang/julia#19800 ...

I can now include PDMP.jl in Julia 0.6 and Julia 0.7 (albeit with loads of deprecation warnings of types vs immutable struct which are not even catered for by Compat.

[tlienart]

5af6eed

[tlienart]

8f541a9 done