Complete support for time in expression

[arbimo]

PR #140 introduced the ability to refer to timepoint in expressions. For instance something like start < start(task0).

Support is currently very minimal as it only allows for LT and LEQ between two time expressions, which are necessary ordering subtasks in hierarchical planning.

To complete the support for time expressions, we should:

• allow usage of more operators on it:
  • equality (TIME/TIME, TIME/NUMBER)
  • addition (TIME/NUMBER)
• have a specific feature flag when it is used in expression such as action conditions (or check and entirely forbid its usage in places where it does not make sense).

Note that the formulation of constraints explicitly mentioning timepoints should get us most of the way in supporting the level of expressiveness required for #58 (just reasoning on timepoints instead of intervals).