Additional ways of reading the goal formula

[francescofuggitti]

Currently, the goal formula can only be specified as a string in the CLI tool and the library.

We would like to have a more general and flexible approach. In particular, the following additional options have been identified:

1. reading from a file
2. reading directly from the (:goal ... specification in PDDL.

An example of the second case is (:goal (once (and a b))) that corresponds to O(a & b)