Allow Stdout / stderr to better participate in operations

[illusional]

Stdout / Stderr are currently types in Janis, but in reality they're probably better as selectors with type File. Being a Selector means they will better participate in operations (like .contents()) or maybe a suggested new .read_json() -> then an index operator could be used.

This would be useful when converting CWL in the cases of ExpressionTool or a commandlinetool that declares the stdout: cwl.output.json (without an outputBinding pattern).

• WDL -> stdout()
• CWL -> File should have path _stdout which you could collect with glob, or manually construct using path / basname

Maybe even making it an operator. You end up the CWL expression of self[0] to represent the glob order from here:

janis-core/janis_core/translations/cwl.py

Lines 1312 to 1318 in 96718bc

	glob, value_from = (
	[STDOUT_NAME, STDERR_NAME]
	if requires_std
	else translate_to_cwl_glob(
	output.selector, inputsdict, outputtag=output.tag, tool=tool, **debugkwargs
	)
	)

class Stdout(Operator):
    @staticmethod
    def friendly_signature():
        return "() -> File"

    def argtypes(self) -> List[DataType]:
        return []

    def evaluate(self, inputs):
        return ""

    def to_wdl(self, unwrap_operator, *args):
        return "stdout()"

    def to_cwl(self, unwrap_operator, *args):
        return 'self[0]'

    def __init__(self, return_type=File):
        super().__init__()
        self.return_type = get_instantiated_type(return_type)

    def returntype(self):
        return self.return_type

    def to_string_formatter(self):
        kwargs = {"stdout": self}
        from janis_core.operators.stringformatter import StringFormatter

        return StringFormatter("{stdout}", **kwargs)