Sets the value of an option.

        :param str key: First identifier of the option.
        :param str subkey: Second identifier of the option.
        :param value: New value for the option (type varies).

        :raise:
            :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                option.
            :ValueError: If the targeted obtion is locked.
            :ValueError: If the provided value is not the expected
                type for the option.
            :ValueError: If the provided value is not in the expected
                available values for the option.

def set_option(self, key, subkey, value):
        """Sets the value of an option.

        :param str key: First identifier of the option.
        :param str subkey: Second identifier of the option.
        :param value: New value for the option (type varies).

        :raise:
            :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                option.
            :ValueError: If the targeted obtion is locked.
            :ValueError: If the provided value is not the expected
                type for the option.
            :ValueError: If the provided value is not in the expected
                available values for the option.
        """
        key, subkey = _lower_keys(key, subkey)
        _entry_must_exist(self.gc, key, subkey)

        df = self.gc[(self.gc["k1"] == key) & (self.gc["k2"] == subkey)]
        if df["locked"].values[0]:
            raise ValueError("{0}.{1} option is locked".format(key, subkey))
        ev.value_eval(value, df["type"].values[0])
        if not self.check_option(key, subkey, value):
            info = "{0}.{1} accepted options are: ".format(key, subkey)
            info += "[{}]".format(", ".join(df["values"].values[0]))
            raise ValueError(info)
        self.gc.loc[
            (self.gc["k1"] == key) &
            (self.gc["k2"] == subkey), "value"] = value