Add arg skip_schema_validation to @on()

[OrangeTux]

The argument can be used to skip validation of a request and response.
It defaults to False, so by default validation is enabled.

The reasoning for this change is that TMH has charge points which use
measurands inside MeterValues messages which are not compliant with the
OCPP 1.6 specification. Therefore the validation of these messages fail.

With this new argument validation of certain messages can be disabled.

Fixes: #54

[ghost]

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[proelke]

Where does the asterisk come into play?

[OrangeTux]

All arguments after the aterisk can only used with as keyword arguments.

So the following call would raise a TypeError:

def foo(*, a):
    pass

foo(1)  # will raise a TypeError

Instead it is required to use keyword arguments:

foo(a=1)  # works fine

Without aterisk the @on() decorator could be used like this:

@on("MeterValues", True)
def on_meter_values(*args, **kwargs):
   pass

Without the keyword it is hard to guess what the meaning is of the second argument. I added the aterisk to make it explicit to disable validation. You can either use skip_schema_validations default value of False. Or, if you want to disable the validation, you can set it to true by keyword arguments:

@on("MeterValues", skip_schema_validation=True)
def on_meter_values(*args, **kwargs):
   pass

[proelke]

Interesting. Thanks for the explanation.