Improve internal documentation

[globusharris]

All functions should have full, clear, commenting with

• full description of valid inputs (including accepted types, which currently is not fully present even in functions with input descriptions)

• full description of accepted output

• citations or link to external documentation when needed

• eventually, test functions to be run w sweave/sphynx

• vignettes for all (non-deprecated) statistics

• [ ] mechanism-bootstrap

• [ ] mechanism-exponential

• [ ] mechanism-gaussian

• [ ] mechanism-laplace

• [ ] mechanism-objective

• [ ] mechanism

• [ ] statistic-covariace

• [ ] statistic-glm

• [ ] statistic-heavyhitters

• [ ] statistic-histogram

• [ ] statistic-mean

• [ ] statistic-tree

• [ ] statistic-variance

• [ ] utilities

• [ ] testing files

[globusharris]

Related: See Issue #38 discussion