moved test_serializable.py to test_utilties/.
Starting to add viscosity at temperature to oil props.
Merge branch 'master' of https://trac.orr.noaa.gov/git/PyGnome
Trac_593
- mass/volume spilled is exposed to webapi and persisted in save files
- set this up so only one is independently set at a time. If user sets
  'mass' then 'volume' is set to None and vice versa. Decided not to
  internally compute one from the other because we may not have density
  and its hard to know what the user set if computing
- updated the name InitMassFromTotalMass to InitMassFromSpillAmount
  since spill amount could be a volume or a mass and the initializer
  should get the correct mass for each element in either case
Trac_593:
- replaced 'mass', 'volume', 'mass_units', 'volume_units' with 'amount'
  and 'units' for the amount of oil spilled as property of Spill object
- kept only the get_mass() method for getting the mass given the
  amount and units of quantity spilled

Also revereted change in InitMassComponentsFromOilProps() so it also
sets the 'mass' array as before. If I simply use the 'mass' array from
the data_arrays, then the initializer for setting 'mass' must be called
before InitMassComponentsFromOilProps()
Fixed bug in outputter where output_timestep of 1 day was crashing
needed to use timedelta().total_seconds() instead of timedelta().seconds
Moved viscosities property from Oil to OilProps.
Added a study of Adios2 initial oil viscosity based on the C++ library code.
Trac_588
- if spill amount is given, default element_type is floating_mass()
- added a test for model run with element_type w/ no initializers
Merge branch 'master' of https://trac.orr.noaa.gov/git/PyGnome
added fields to component mover schema and a component mover to test_persist
Merge branch 'master' of https://trac.orr.noaa.gov/git/PyGnome
Added a viscosity-at-temperature method to OilProps.