Add stride parameter for logfile output variables

[SteveMaas1978]

The plot_stride parameter allows users to specify how often to write the model's state to the plot file.

A similar feature for log file output would be useful.

[inkandvijay]

This feature would be useful, especially when working with 'dynamic' simulation with very small time-steps. I need only logs at specified time, whereas right now it prints every step. The difference in runtime is a lot, it takes >x10 time it takes for run without generating log files.