set to 0
- type correction

Related to #1355

Also adds unit test. Closes #1318

Also adjusts unit tests accordingly. Closes #1332

Prevents general overwriting of results in oo_ and having stale results

An exogenous variable appearing in t+1 (as productivity in the expected
return of capital in the Euler equation) is not a forward variable.

Interface was still missing and documentation of option was in wrong place in manual and wrong

Manual: add that if several files differ only by the extension, the filaname must include the extension and be written between quotes

- adds diffuse_filter as option of calib_smoother
- adds test case for calib_smoother with diffuse_filter