Pyromat

- Modified `ig`, `ig2`, and `igmix` classes to accept enthalpy and entropy simultaneously (github issue 83)
- Added entropy of mixing to the `igmix` s() calculation (github issue 92)
- Added the `igtools` module with dynamic ideal gas mixture support
- Added the `ismass()` function in the units module
- Added `sid()` to the `__basedata__` class (all data instances)
- Added `hill()` to the `__basedata__` class
- Added `def_p_unit` and `def_T_unit` to the configuration system (github issue 68)
- Added `def_p()` and `def_T()` as methods to the `PMConfig` class to automatically handle the default units
- Added `a()` speed of sound method to all classes
- Corrected the error in the R1234ze saturation line data (github issue 86)
- Added Helmholtz (`f`) and Gibbs (`g`) energies to all classes
- Wrote the _ds() precision inner saturation property method; fully functional, but not yet used.

- Corrected a bug reported in issue 64 where inverse routines were not returning the correct units.

- Updated the README to adopt recommendations made by the JOSS community - specifically to include recommendations for community involvement.
- Added the optional `pip install pyromat[dev]` option, which requires the `pytest` package.

- Changed the call signature for `T_s`, `T_h` functions to address issue 52
*NOTE* inverse methods like `T_s` and `T_h` are now deprecated.
- Reassessed all multiphase `dlim` values in the core data. This addresses issues 44, 45, and 46.

- Issued bugfixes for github issues 41, 42, 43
- Corrected a boneheaded typo in igmix that should have been caught in testing.
- Reverted to `_tditer()` in `mp1._T()`
- Eliminated the upper 1% grace range in `Ts()` to prevent imaginary values past Tc.

- Corrected small errors in mp1 model properties near the critical point.
This bug resulted in strange things like negative cp values, but it was only found VERY close to the critical point. Otherwise, errors were so small that the properties still passed validation checks against reference data.
- Added iteration to the pyromat configuration class
- Added the specific volume property to all classes
- Added the warning_verbose configuration entry to allow users to mute warning messages
- Transitioned to a fully flexible property argument format that accepts combinations of h,s,e,T,p,d,v,x.
- Obsoleted "inverse" property methods (like T_s and T_h) - they are still available for reverse compatibility.
- Changed the out-of-bounds detection behavior: now returns config["def_oob"] (default np.nan) on those array elements that are out-of-bounds.
- Added O2, R1234ze to the multiphase collection
- Corrected a number of minor bugs reported since the last release. These are documented on the PYroMat github issues page.