Parameters 'temp_dir_root', 'fail_dir_root' and 'cache_dir' not handled properly

[rambausek]

User set directories not expanded when setting these parameters via

dolfin.parameters['form_compiler']['temp_dir_root'] = os.path.abspath('./temp_dir_root')

'cache_dir', 'temp_dir_root' and 'fail_dir_root' are not expanded (os.path.expanduser...) correctly.

The corresponding code in 'validate_params' is never reached, since the defaults are of type 'None' and not 'str'.