Update GCOV and GSLC Specification XMLs (isce-framework#70)

* disable polarimetric symmetrization by default

* revert changes to `symmetrize_cross_pol_channels`

* Update GCOV and GSLC specification XMLs

* fix unit test by removing dots from projection attributes

* Update GCOV and GSLC specs XMLs for R5.0

* Update GCOV and GSLC specs XMLs for R5.0

* Add new attributes `isReceiveOnlyChannel1` and `isReceiveOnlyChannel2` to unit test

* Attributes `isReceiveOnlyChannel1` and `isReceiveOnlyChannel2` are string

* Add new attributes `isReceiveOnlyChannel1` and `isReceiveOnlyChannel2` to unit test

Author: gshiroma