Add additional example .mol files to inst/extdata

[Aariq]

• Replaces and adds .mol files to inst/extdata as discussed in #79
• Adds the names of the example compounds to the documentation for mol_example()

[Aariq]

Before I merge this, would it be better if mol_example() just returned a (named) vector of file paths? E.g.

mol_example()
#> [1] "/Users/ericscott/Documents/GitHub/volcalc/inst/extdata/C00031.mol"
#> [2] "/Users/ericscott/Documents/GitHub/volcalc/inst/extdata/C08491.mol"
#> [3] "/Users/ericscott/Documents/GitHub/volcalc/inst/extdata/C16181.mol"
#> [4] "/Users/ericscott/Documents/GitHub/volcalc/inst/extdata/C16286.mol"
#> [5] "/Users/ericscott/Documents/GitHub/volcalc/inst/extdata/C16521.mol"

Then you could do things like calc_vol(mol_example()) @KristinaRiemer

[Aariq]

possibly add readLines() to example for mol_example()