Improve get functions for patterns

[mmatera]

This is one tiny change I wanted to implement for a while. Im WMA, if you want to implement a rule that returns the same expression if certain test fails, the way to do it is

expression: Half[expr]:=If[NumberQ[expr], expr/2, expression]

i.e. use a Pattern[patname, pat] expression to pass the whole original unevaluated expression (Half[a] in the example) to the replace expression. However, we could not do this in the patterns especified in docstrings, because we "overloaded" the meaning of patname: pattern in the function that loads eval_* and format_* methods: we use them to indicate in the case of format_ methods to which Format corresponds the rule.
As in some methods we need the full original expression, then we added a "hack" to interpret a parameter expression in an eval_ function to contain the whole expression.

This PR removes the need of all this hackiness by changing the convention in format_ methods docstrings: if you want to especify an specific Format, you can do it with a docstring of the form (form1, form2,...,): exprpattern which is not a valid WL expression.

We can also use this form to avoid the use of Optional in the eval_ methods, so each eval_ method could return always a BaseExpression. But this will come later.

[rocky]

LGTM