Fix issues with geotiff conversion and add rotation

[brenmous]

• Add rotation option
• Add specifying depth slices by depth in metres
• Add log scaling option
• Fix bug with calculation of geotiff origin
  • Was shifted 1/2 cell south instead of north
• Fix bug in calculating padding when reading model file
  • East padding was off-by-one and north padding was being set to the wrong attribute
• Fix bug with trimming of padding from model grid
  • X and Y axes were mixed up, causing a mismatch between grid points and values to be interpolated
  • This fix now allows you to specify different values for xpad/ypad
• Refactor and test conversion module