ENH: relocate WM_PROJECT_SITE default (issue #1050)

- was PREFIX/site, now PROJECT/site

  This avoids several issues when installing OpenFOAM in clusters
  without an intermediate OpenFOAM-specific installation prefix.

  The 'site' directory may have a reserved meaning in these situations
  and it is undesirable to 'leak' upwards into the parent directory to
  look for configuration files.

  Placing the default within the project directory avoids this.
  Alternative locations can be given via the WM_PROJECT_SITE variable.