feat!: change Optional to IsOptional

[ZanSara]

Related to #105 and #111

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So far in Canals we've used the type Optional to imply that a specific input value was not to be waited for when a component is ready to run. However, this is in conflict with the actual semantics of Optional, which refers to an input value that can be either of a type or None.

This PR introduces a new type. IsOptional, to separate the two concepts.

Note that IsOptional is an alias of Optional, so mypy may believe that the value can become None. On the other hand, if we don't use IsOptional as an alias of Optional but rather a "no-op" type, some parameters need to be typed this way:

param: IsOptional[Optional[int]] = None

which looks quite odd. Open to suggestions on this regard. A better name for the type might do the trick.

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Note. This PR is BREAKING. Components need to be adapted, as with this change all inputs will become mandatory.

OLD SYNTAX:

def run(self, mandatory: int, not_mandatory: Optional[int] = None)

NEW SYNTAX:

def run(self, mandatory: int, not_mandatory: IsOptional[int] = None)

[silvanocerza]

This seems even more counter intuitive and confusing that having to specify a default when declaring run().

There's also a difference between Optional and settings a default argument. Optional refers to the type of the argument and should not be mixed up with the optionality of an argument.

Optional tells the user that the value can be None but not that they omit that input.
Defaulting the argument tells the user that they can omit the input, but it doesn't necessarily mean it can be None.

Clashing the two together changes the behaviour of optional inputs too.

This tells me that I can omit the foo input since it has a default, but it still tells me it must be an int.

def run(foo: int = 10)

This instead tells me that I can omit the foo input, but it can also be None. That is not semantically clear. And it's also confusing as it's way too similar to Optional.

def run(foo: IsOptional[int] = 10)

In the end I believe we must not do this.

[ZanSara]

Yep I eventually reached the same conclusion. I still believe our use of Optional is wrong, but so is this approach. I am going to open another PR using the default value as an indicator instead.