[Graph]: make sure #threads == 2^n.
[SAFS]: throw exception if a SAFS file doesn't exist.
[Graph]: getting params in R binding doesn't need init.
cmake compiles code with -fPIC.
[Graph]: improve Makevars for R bindings.
Merge branch 'release-v0.1' into release
Merge branch 'graph-release' of github.com:icoming/FlashGraph into release