added new funciton inv to stan
added new function inv_sqrt
added new function inv_square
Merge branch 'develop' into feature/adding_inv_functions
added inv, inv_sqrt, inv_square to doc
fixed inv, inv_sqrt, inv_square functions and added boundary tests
refined boundary tests for inv, inv_sqrt, inv_square
removed redundant operations in inv_sqrt, inv_square, pow
Merge branch 'develop' into feature/adding_inv_functions
Merge pull request #92 from stan-dev/feature/adding_inv_functions

Feature/adding inv functions