add stats, stats_1d, stats_2d to keep statistics during simulations

[jcurtis2]

Implement the contents of stats.F90 to store averages and variances of scalar values, 1d arrays and 2d arrays such that ensembles of simulations can be run. These types have the advantage of computing the mean and variance on the fly rather than storing all data from all runs and computing it as a final step.

These will also replicate how we process netCDF output in fortran processing programs like urban_plume_process.F90.

• [ ] stats_clear
• [ ] stats_1d_clear
• [ ] stats_2d_clear
• [ ] stats_add
• [ ] stats_1d_add
• [ ] stats_2d_add
• [ ] stats_1d_add_entry
• [ ] stats_2d_add_entry
• [ ] stats_2d_add_col
• [ ] stats_2d_add_row