Adding custom number format to a template could fail with existing non-sequential ids

[jahav]

Adding custom formats to a template that already contained the custom number formats could fail, if the number format ids in the template weren't sequential.

E.g. in this sequence, the 169 is missing.

<numFmts count="7">
	<numFmt numFmtId="165"
			formatCode="[$-409]dd\-mmm\-yyyy;@"/>
	<numFmt numFmtId="166"
			formatCode="[$-409]hh:mm:ss;@"/>
	<numFmt numFmtId="167"
			formatCode="[$-409]hh:mm;@"/>
	<numFmt numFmtId="168"
			formatCode="[$-409]hh;@"/>
	<numFmt numFmtId="170"
			formatCode="#,##0.00;\-#,##0.00;0;@"/>
	<numFmt numFmtId="171"
			formatCode="[$-409]dd\-mmm\-yyyy\ hh:mm:ss;@"/>
	<numFmt numFmtId="172"
			formatCode="#,##0;\-#,##0;0;@"/>
</numFmts>

Resolves #1679