WIP: improving fgmax_tools

Improving fgmax_tools.py to have a single class `FGmaxGrid` for creating or reading input file, and also for reading the output file generated by GeoClaw for plotting.  

Is there a better name for this class?

Still need to add tests.

Should move tools for making color maps to `geoplot.py`.  Add other plotting tools?

Also fixed bug in how extent was being set in `Topography.read_header`