Create a function to store the local exceedence intensity maps as netcdf file

[simonameiler]

It would be nice to have a new function to store the hazard maps that are generated in the Hazard.plot_rp_intensity() function as netCDF files.

My suggestion is incorporated in the following branch and contains one additional function (write_local_exceedence_inten_netcdf) in the hazad.base class.

What do you think? Shall I create a pull request or do you have suggestions for improvement?

[peanutfun]

Related to #1035 #905