fixing auto dt sizing function

[krober10nd]

The automatic selection of the numerically stable timestep for which to CFL limit the mesh sizing function should be based on the max_cr not the min_cr.

This appears to fix #114 with a mesh that should be numerically stable at a timestep of roughly 1.34 seconds because of the high resolution zone near New York, e.g.,

Enforcing timestep of 1.1134 seconds with Courant number bounds of 2.2204e-16 to 0.5

The outer patch by itself is theoretically stable with:

Enforcing timestep of 6.2979 seconds with Courant number bounds of 2.2204e-16 to 0.5

[WPringle]

Hey, yes the fix seems correct.
There is this case where you choose obj.dt to be length 2 and maxCr will be infinity. I think this is mainly user problem but maybe we could have an error for length(obj.dt) = 2 and dt == 0? Saying that dt = 0 is only intended for restricting the upper bound of Cr?

                elseif length(obj.dt) == 2
                    % minimum Courant number bound 
                    minCr = obj.dt(2); 
                    maxCr = inf; 
```

[krober10nd]

like this?

            % Determine Courant min. and max. bounds
                 if length(obj.dt) < 2
                     % default behavior if no bounds are passed
                     % for explicit type schemes we ensure the maxCr is
                     % bounded
                     minCr = eps; % Cannot be zero!!!
                     maxCr = 0.5;
                 elseif length(obj.dt) == 2
                     % minimum Courant number bound
                     minCr = obj.dt(2);
                     maxCr = inf;
                 elseif length(obj.dt) == 3
                     % minimum and maximum Courant bound
                     minCr = obj.dt(2);
                     maxCr = obj.dt(3);
                 elseif length(obj.dt) == 2 && obj.dt(1)==0.0
                     error('dt = 0 is only intended for restricting the upper bound of Cr')
                 else
                     error('Length of dt name-value pair should be <=3')
                 end

[WPringle]

Yeah except that wouldn't work because it would go into the elseif length(obj.dt) == 2 above, so like this:

            % Determine Courant min. and max. bounds
                 if length(obj.dt) < 2
                     % default behavior if no bounds are passed
                     % for explicit type schemes we ensure the maxCr is
                     % bounded
                     minCr = eps; % Cannot be zero!!!
                     maxCr = 0.5;
                 elseif length(obj.dt) == 2
                     if obj.dt(1)==0.0 
                         error('dt = 0 is only intended for restricting the upper bound of Cr')
                     end
                     % minimum Courant number bound
                     minCr = obj.dt(2);
                     maxCr = inf;
                 elseif length(obj.dt) == 3
                     % minimum and maximum Courant bound
                     minCr = obj.dt(2);
                     maxCr = obj.dt(3);
                 else
                     error('Length of dt name-value pair should be <=3')
                 end

[krober10nd]

ah yea. cool.