0.1.5 release

add conversion functions + refactor (#9)

- add **moles2grams(formula, moles)**
- add **grams2moles(formula, grams)**
- add example.
- add guarding with **size** parameter in **weight()**
- improved usage **abbrev**(name) versus **el**(index).
- improved readme.md.
- add define for **ATOMIC_WEIGHT_MAX_SPLIT_LIST**
- update keywords.txt.
- minor edits.