Draft: kth-order approximation of conditional welfare; issue #1680

Partially addresses issue #1680 (closed):

• unconditional welfare resorts to dynare++ simulation tools, which shall be updated very soon
• matlab '+objective' files are not generated for orders>3.

TO DO:

• implement a function computing kth-order approximation of simulated moments of y
• amend the preprocessor to generate '+objective' files for orders>3