Add new “log” option to “var” statement

When an endogenous is declared with “var(log)”, say “y”:
– creates an auxiliary named “LOG_y”
– replaces “y(±l)” everywhere by “exp(LOG_y(±l))”
– adds a new auxiliary equation “y=exp(LOG_y)”
– adds a new definition “LOG_y=log(y)” in set_auxiliary_variables.m and
  dynamic_set_auxiliary_series.m files

This option also works in conjunction with “deflator=…”, such as “var(log,
deflator=…)” (the “log” must appear befor “deflator”). There are no provisions
for combining “log” with “log_deflator”, because that would not make much sense
from an economic point of view (amounts to taking the log two times).

Ref. dynare#349