Add mixed mode interpolation increment option

[gaben]

What steps will reproduce the problem?

1. Have individual house numbers like 1/a, 1/b, 2/a, 2/b, 3/a and so on
2. Try to tag them with interpolation

What is the expected result?

The plugin has an increment option for sequence length (for the example above it's 2) and can interpolate addresses based on the rule.

What happens instead?

No option for increments, an error message pops up saying something about address validity. It can't handle "mixed" addresses.

Please provide any additional information below. Attach a screenshot if possible.

URL:https://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2021-12-02 09:54:09 +0100 (Thu, 02 Dec 2021)
Build-Date:2021-12-03 02:31:02
Revision:18327
Relative:URL: ^/trunk

Identification: JOSM/1.5 (18327 hu) Windows 10 64-Bit
OS Build number: Windows 10 Pro for Workstations 2009 (19044)
Memory Usage: 923 MB / 7266 MB (422 MB allocated, but free)
Java version: 1.8.0_311-b11, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM

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