Merge pull request #510 from dbetebenner/master

Getting up to date with master and tweaking with parameters in studentGr...
Add ability to use pre-existing SIMEX coefficient matrices
Add missing parens
Add simex.use.my.coefficient.matrices to internal call to .simex.sgp
move simex.use.my.coefficient.matrices check inside k and L loops.
Fix simex.coef.matrices assignment when using pre-existing matrices.  remove 'z' index
Fix errors in finding/using pre-existing SIMEX matrices
Run .get.percentile.predictions in parallel
Merge pull request #511 from adamvi/master

Add ability to use pre-existing SIMEX coefficient matrices
Merge branch 'master' of https://github.com/CenterForAssessment/SGP