Freezerbox

Feature
* Add the *keys* argument to `iter_combos()` ([`d93d239`](https://github.com/kalekundert/freezerbox/commit/d93d239dfd07099a472e92efa63c29ada3ff5fe3))

Feature
* Parse quantities with or without units ([`c6251d5`](https://github.com/kalekundert/freezerbox/commit/c6251d531e61220eb952bc5e75c2160cc52c0bcc))

Feature
* Infer molecular weight from length ([`94a2d80`](https://github.com/kalekundert/freezerbox/commit/94a2d801523a8f4833a77266dfd09bbb5abec3d2))

Feature
* Allow conversions to/from µg/µL ([`c8e46b1`](https://github.com/kalekundert/freezerbox/commit/c8e46b1789e9712a469d6cd172177e47824fe5d9))

Fix
* Respect empty databases ([`c81a511`](https://github.com/kalekundert/freezerbox/commit/c81a5112b05452a7be9d360f6ff152b45b478810))

Feature
* Add dependency getter to `Reagent` class ([`fb40b17`](https://github.com/kalekundert/freezerbox/commit/fb40b170a6d5668e172b662cd60135ecc9c59bb5))
* Add `parse_mass_ug()` ([`37a4e4c`](https://github.com/kalekundert/freezerbox/commit/37a4e4c229cec697f98200caef1ed3350d53477c))