add deprecate file (qutip#274)

Author: ytdHuang

src/QuantumToolbox.jl

@@ -102,4 +102,7 @@ include("metrics.jl")
 include("negativity.jl")
 include("steadystate.jl")
 
+# deprecated functions
+include("deprecated.jl")
+
 end

src/deprecated.jl

@@ -0,0 +1,15 @@
+#=
+This file gathers all the deprecated names (structures, functions, or variables) which will be removed in the future major release.
+
+- Before the major release, the deprecated names will just throw errors when they are called.
+- If the deprecated names were once exported, we will still export them here until next major release.
+- If we decide to push a major release, cleanup this file.
+
+Example:
+
+export deprecated_foo
+
+function deprecated_foo(args...; kwargs...)
+    error("`deprecated_foo` has been deprecated and will be removed in next major release, please use `new_foo` instead.")
+end
+=#