(WIP) working on OilProps needed for evaporation
- moved get_oil, oil_from_density functions to oil_library name space
- also moved the _sample_oils dict to here
- added functions in oil_props for computing boiling_point,
  vapor_pressure_ratio, molecular_weights for saturates
  - added functions to facilitate testing
- STILL NEED TO ADD TESTS
(WIP) evaporation
- added _exp_decay function that take the exp(lambda * time) form and
  gets the _half_life factors before calling the _halflife() function
Merge branch 'master' of https://trac.orr.noaa.gov/git/PyGnome into evaporate
Merge branch 'master' of https://trac.orr.noaa.gov/git/PyGnome into evaporate
(WIP) refactored get_oil, oil_from_density convenience functions to
oil_library namespace
- reverted the change of OilProps so it contains the Oil object instead
  of deriving from it. It was cumbersome to derive so left this alone
  for now
(WIP) updated py_gnome tests with oil_library updates
added a check to dagtree search to look at neighbors if lost
Merge branch 'master' of https://trac.orr.noaa.gov/git/PyGnome
Added a new Oil Library database object: Category