Grids: module for grid specifications, scaling and netcdf IO

• mo_grid:
  • added grid_t class to hold a uniform grid definition
    • single cell-size
    • Cartesian and Spherical grid support
    • aux-coordinates support
    • support for periodic spherical grids (along lon-axis)
    • NetCDF in/out routines
    • ascii grid in/out routines
    • aux-coords upscaling support (lat-lon coordinates estimated from fine grid by averaging)
    • derive coarse/fine grids based on target resolution or scaling factor
    • supports flipping y-axis (top-down or bottom-up)
• mo_grid_io:
  • handlers for netcdf files based on a grid (input_dataset, output_dataset)
  • uses datetime to select data on time dimension
• mo_grid_scaler:
  • regridder implementation based on grid type
  • can be used for up and down scaling
  • wide range of scaling operators provided
• mo_message:
  • add warn_message as 3rd option
• mo_utils:
  • add flip and flipped for sp/dp/i4/i8 for 1-4 dims
• mo_datetime:
  • added decode_cf_time_units: convert cf time units to a reference datatime and a timedelta ("hours since 1990-06-01 12:00")
  • added delta_from_string: cf string for time delta (like "hours") to timedelta (like timedelta(hours=1))
  • added next_new_<time_frame> methods to get next new year/month/week/day/hour/minute
• general:
  • added FORCES_EXE option to CMake to compile a singe executable linked against forces
  • added examples folder