Fixes around histval and loglinear

• Allow using histval to set initial condition for Kalman filter
• Improve compatibility of loglinear with forecast and conditional_forecast-commands (also improves documentation)
• Expands respective integration tests

NB:

1. Design philosophy: the histval-block specification should be independent of whether loglinear is used or not (as is the case with e.g. initval or steady_state_model). This requires Dynare taking the log automatically when required. This clashes with conditional_forecast_path-block, where the specified controlled values are never transformed.
2. These changes do not affect the ramsey-command, which seems to in principle allow for loglinear and histval. Should we try to make it compatible or provide an error?