Support of StandardInputNames in XML

[BGWKlein]

In HydPy it is possible to use StandardInputNames for the meteorological input by setting

pub.sequencemanager.convention = "HydPy"

When running HydPy by passing a XML configuration file, this is not possible.

It would be great to include this option to XML support, e.g. by adding the optional element convention as child element to reader (Standard 'model-specific'):

            <reader info="all input data">
                <selections>from_keywords</selections>
                <sequences>
                    <evap_pet_hbv96>
                        <inputs>
                            <normalairtemperature/>
                            <normalevapotranspiration/>
                        </inputs>
                    </evap_pet_hbv96>
                    <hland_96>
                        <inputs>
                            <p/>
                            <t/>
                        </inputs>
                    </hland_96>
                </sequences>
                <filetype>nc</filetype>
                <mode>jit</mode>
                <convention>HydPy</convention>
            </reader>

The only unclear question is wether to add model specific names (I would prefer this) or to add standard input names when specifying the sequences of the individual models.