Undo setting PCE properties as protected

For the MetaModel class split some of the PCE-specific properties such as pce_deg, polycoeffs, clf_poly and more were set to be protected. Since some of these should be set by the user (e.g. pce_deg, pce_reg_method), these should be set public again.

Change this for the following properties:

• _pce_reg_method
• _pce_deg
• _pce_q_norm

Properties to discuss:

• _pce_reg_options
• _coeffs_dict

This affects the following classes

• PCE
• PostProcessing
• Engine (?)