PICA Path: support selection by counter in subfield `x`

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Support selection by counter in subfield x (e.g. 209Ax00 $a) as provided by WinIBW

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Syntax clashes with abbreviated subfield selection (209Ax0 = 209A$x0) but an exception could be made for subfield x in level 2. With syntax extension we could do it with subspecs, e.g. 209A{$x=00} and 209A{$x} but full syntax is rather complex. Simple (non-)existence (209A{!$x}) and value equivalence could be supported.

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requires #108