Enhancement: Native IPHREEQC support?

[rkingsbury]

phreeqpython, used by the native and phreeqc modeling engines, ships compiled VIPHREEQC libraries and provides a python wrapper to access them. These are both maintained by Vitens and build upon official IPHREEQC modules released by USGS.

Given that pyEQL only needs a few very specific capabilities of PHREEQC, it may be beneficial to access IPHREEQC directly (mainly to make it easier to track upstream updates). The modeling engines we use really only need access to speciation, activity coefficients, volume / density, and possibly saturation indices (see #58 ). Everything else is provided natively via Solution methods. This transition would also allow us to drop phreeqpython as a dependency.