add function examples

Function examples should:
- Be present in every roxygen2 #' block
- Include ~2-4 examples
- Demonstrate common use cases and important options
- Include library() calls or otherlib::fun() syntax to manage dependencies
- Only use packages that are also listed in the Imports or Suggests blocks of the DESCRIPTION file
- Either run quickly and without warning/error, or be wrapped in a \dontrun{} block
- Use only data available within the package and/or reproducibly generated (e.g., through fixed text definitions of the data, or using set.seed() before random data generation)
- Run cleanly on package check (Ctrl-Shift-E)

To do:
- [x] aggregateSolute (#42)
- [ ] covariance (#31)
- [ ] diagnostics (#30)
- [ ] getResiduals (#41)
- [ ] interpolations (#35)
- [x] leverage (#45)
- [ ] loadComp (#39)
- [ ] loadflex (#47)
- [ ] loadInterp (#36)
- [ ] loadLm (#34)
- [ ] loadModel (#33)
- [ ] loadModelInterface (#40)
- [ ] loadReg (#37)
- [ ] loadReg2 (#38)
- [x] match.arg.loadflex (#26)
- [ ] metadata (#32)
- [ ] outliers (#44)
- [ ] plots (#46)
- [ ] predictors (#43)
- [x] text (#27)
- [x] uncertainty (#29)
- [x] unit.conversions (#28)