New method phase, several changes.

New features
* Method shift: now skips computations for zero offset.
* New method phase: computation of phase among two signals.

Changes
* Method split: now manage a split time outside signal domain returning the
  proper void signal.
* Method correlation: dropped step_left and step_right arguments, substituted by
  skip and width.
* Method plotchar: dropped period argument.
* Method mod2code: now symbol start time is the phase with respect to the signal
  start time.
* Methods noise and square: now require an origin argument.

Internals
* Method correlation: refactoring for new arguments skip and width.
* Method correlation: augumented test.
* New method phase: add test.

Documentation
* Started better layout for function/methods arguments and return patterns.
Documentation update.