v0.5.3

• function phertz now has additional argument to indicate if mean or maximum
  pressure should be returned
• clarified that Hertz equations are only valid for elliptical and circular
  contacts; equations for line contacts are not implemented at the moment
• added additional test cases for Hertz equations
• fixed bug in Hertz pressure calculation that may lead to wrong results for
  elliptical contacts if one of the contact bodies is completely flat
• module rough_surfaces added