ENH: Consistency check of Ehepartner and Einstandspartner

[MImmesberger]

We should check that Ehe- and Einstandspartner specified via p_id_ehepartner and p_id_einstandspartner are consistent. Two people can be Einstandspartner, but not Ehepartner. The other way around is not possible, i.e. the following is okay:

p_id	p_id_ehepartner	p_id_einstandspartner
0	-1	1
1	-1	0
2	-1	-1

But this is not:

p_id	p_id_ehepartner	p_id_einstandspartner
0	1	-1
1	0	-1
2	-1	-1

and also not:

p_id	p_id_ehepartner	p_id_einstandspartner
0	1	-1
1	0	0
2	-1	1