[read] read shared formulas. closes #1072

[JanMarvin]

First attempt to fix #1072

[JanMarvin]

it might come in handy one day, to move this into a custom function to alter cc in the workbook, because IIRC there is an issue, if we write onto a cell that is part of a shared formula, hence if this works, we should be able to convert the shared formula cell into a non shared formula cell first, that we can afterwards write over safely.