Add explicit constructors for the timesteppers (#4175)

* change to the timestepper

* fix the constructor

Author: simone-silvestri

src/TimeSteppers/TimeSteppers.jl

@@ -20,27 +20,6 @@ Abstract supertype for time steppers.
 """
 abstract type AbstractTimeStepper end
 
-"""
-    TimeStepper(name::Symbol, args...; kwargs...)
-
-Returns a timestepper with name `name`, instantiated with `args...` and `kwargs...`.
-
-Example
-=======
-
-```julia
-julia> stepper = TimeStepper(:QuasiAdamsBashforth2, CPU(), grid, tracernames)
-```
-"""
-function TimeStepper(name::Symbol, args...; kwargs...)
-    fullname = Symbol(name, :TimeStepper)
-    TS = getglobal(@__MODULE__, fullname)
-    return TS(args...; kwargs...)
-end
-
-# Fallback
-TimeStepper(stepper::AbstractTimeStepper, args...; kwargs...) = stepper
-
 function update_state! end
 function compute_tendencies! end
 
@@ -60,4 +39,31 @@ include("quasi_adams_bashforth_2.jl")
 include("runge_kutta_3.jl")
 include("split_hydrostatic_runge_kutta_3.jl")
 
+"""
+    TimeStepper(name::Symbol, args...; kwargs...)
+
+Returns a timestepper with name `name`, instantiated with `args...` and `kwargs...`.
+
+Example
+=======
+
+```julia
+julia> stepper = TimeStepper(:QuasiAdamsBashforth2, CPU(), grid, tracernames)
+```
+"""
+TimeStepper(name::Symbol, args...; kwargs...) = TimeStepper(Val(name), args...; kwargs...)
+
+# Fallback
+TimeStepper(stepper::AbstractTimeStepper, args...; kwargs...) = stepper
+
+#individual contructors
+TimeStepper(::Val{:QuasiAdamsBashforth2}, args...; kwargs...) = 
+    QuasiAdamsBashforth2TimeStepper(args...; kwargs...)
+
+TimeStepper(::Val{:RungeKutta3}, args...; kwargs...) = 
+    RungeKutta3TimeStepper(args...; kwargs...)
+
+TimeStepper(::Val{:SplitRungeKutta3}, args...; kwargs...) = 
+    SplitRungeKutta3TimeStepper(args...; kwargs...)
+
 end # module