Add a simplified named accessor for the most common use case

[xparq]

The current generic one has an array (std::vector) for every named argument, because they may have multiple values.
But they almost never do...

So, there should be a getter to receive a simple map<string, string>
a) in case no args have multiple values,
b) or even if some do, they could just be omitted, by some explicit request (e.g. a different call, or a param.).

OTOH, this feels like such a superficial issue: why would (99% of) anyone not be done with just the direct [] and () named accessors?...