Fill values not detected

**Describe the bug**
DEMs created with for instance ArcPro have a fill value set to 1e+38. This value is not picked up by `ncread` and then can negatively impact interpolation of topobathymetry to the mesh vertices. 

**Proposed fix** 
Put in a logic in `GridData` to catch abs(bathymetry) values larger than Mariana's trench and set them to NaN. 
