Add GM3 and GMM properties to TDEheader

These GM values are not included into DE directly. But they can be calculated using values GMB and EMrat.

`DExxx` modules in JDEread does not set these values, so at porting stage it will be forgotten to add them. To avoid it, I have created private functions `getGM3` and `getGMM` who calculates `fGM3` and `fGMM` values when properties are being read for first time. Every next call will return cached value.