Bumped version to 9.0.0-a7 and updated Changes file
Merged: Fixed precision issue with computing the projection matrix for dsm depth remapping, which was causing deep holdout problems.

Conflicts:
	src/IECoreRI/SHWDeepImageReader.cpp
Merged: Applied same fix to shw writer as reader.
Hacked up constants in test case to pass again.

Conflicts:
	src/IECoreRI/SHWDeepImageWriter.cpp
Merge pull request #347 from danieldresser/shwPerpFix9

Applying shw conversion precision fixes to master