fix(dxf): a DIMSTYLE record inherits the header value of an omitted code

[simonedd]

The DIMSTYLE record is now seeded with the header variables so that only the codes actually present in the file overwrite them.

[DomCR]

Hi @simonedd,

You are setting the dimensions using the override, not the default dimension, I'm not sure that this is the right behavior.

[simonedd]

Hi @DomCR,

I think the property name is misleading here: CadHeader.DimensionstyleOverrides is not a DSTYLE
override, it is the storage of the header dimension variables themselves. I use them as default
values instead of the hard-coded DimensionStyle ones.

My case: a DXF whose DIMSTYLE record omits 78 (DIMZIN) and 272 (DIMTDEC), while the header has $DIMZIN 0 and $DIMTDEC 4. We read those two back as 8 and 2, the class hard-coded defaults; ODA reads them as 0 and 4.