[NEWS] New syntax for time_step

In numerical_parameter, time_step requires fixed or adaptative keyword.
cfl keyword is replaced by cfl_factor and must be place in the adaptative block.

Syntax:

# Fixed time step
time_step fixed DOUBLE_EXPRESSION;

# Adaptative time step
time_step adaptative {
    # CFL factor
    cfl_factor DOUBLE_EXPRESSION;
    # First time step
    first_step DOUBLE_EXPRESSION;
    # [OPTIONAL] Minimal time step (default: 0d0)
    min_step DOUBLE_EXPRESSION;
    # [OPTIONAL] Maximal time step (default: huge(1d0))
    max_step DOUBLE_EXPRESSION;
    # [OPTIONAL] Maximal increment between two time steps (default: huge(1d0))
    max_increment DOUBLE_EXPRESSION;
    # [OPTIONAL] Maximal ratio between two time steps (default: huge(1d0))
    max_ratio DOUBLE_EXPRESSION;
}