[Julia API] Initialize output variables

In the Julia API output int, double and string were not initialized and, hence, we passed undefined variables to ccall in a couple of places. This PR includes the following:

• First commit initializes ints with Ref{Cint}() and returns the referenced value. Example diff:

   function getElement(elementTag)
  +    api_elementType_ = Ref{Cint}()
       api_nodeTags_ = Vector{Ptr{Cint}}(1)
       api_nodeTags_n_ = Vector{Csize_t}(1)
       ierr = Ref{Cint}()
       ccall((:gmshModelMeshGetElement, gmsh.clib), Void,
             (Cint, Ptr{Cint}, Ptr{Ptr{Cint}}, Ptr{Csize_t}, Ptr{Cint}),
  -          elementTag, elementType, api_nodeTags_, api_nodeTags_n_, ierr)
  +          elementTag, api_elementType_, api_nodeTags_, api_nodeTags_n_, ierr)
       nodeTags = unsafe_wrap(Array, api_nodeTags_[1], api_nodeTags_n_[1], true)
       ierr[] != 0 && error("gmshModelMeshGetElement returned non-zero error code: $(ierr[])")
  -    return elementType, nodeTags
  +    return api_elementType_[], nodeTags
   end

• Second commit initializes output double with Ref{Cdouble}() and returns the referenced value. Example diff:

   function getNumber(name)
  +    api_value_ = Ref{Cdouble}()
       ierr = Ref{Cint}()
       ccall((:gmshOptionGetNumber, gmsh.clib), Void,
             (Ptr{Cchar}, Ptr{Cdouble}, Ptr{Cint}),
  -          name, value, ierr)
  +          name, api_value_, ierr)
       ierr[] != 0 && error("gmshOptionGetNumber returned non-zero error code: $(ierr[])")
  -    return value
  +    return api_value_[]
   end

• Third commit initialized output string with Ref{Ptr{Cchar}} and returns the referenced pointer as a Julia String. Example diff:

   function getPhysicalName(dim, tag)
  +    api_name_ = Ref{Ptr{Cchar}}()
       ierr = Ref{Cint}()
       ccall((:gmshModelGetPhysicalName, gmsh.clib), Void,
             (Cint, Cint, Ptr{Ptr{Cchar}}, Ptr{Cint}),
  -          dim, tag, name, ierr)
  +          dim, tag, api_name_, ierr)
  +    name = unsafe_string(api_name_[])
       ierr[] != 0 && error("gmshModelGetPhysicalName returned non-zero error code: $(ierr[])")
       return name
   end