Improve gnuplot and VTK writers.

• allow output without time specifier
• fix gnuplot variable order

gnuplot is meant to write variables in the order given in the script file. However, the order is incorrect when mixing variable types. For instance in 1d, if requesting hydro variables rho, u and p in this order, the output will be rho, p and then u. The first, the two variables are valued in R and then the variable valued in R^1.