Calkulate

***Bug fixes***

* Fix bug in handling `data["file_good"] = False` cases.
* Set DIC to zero where its value is NaN.
* Skip over errors in titration files with a warning rather than throw a breaking error.

* Added `convert` module with convenience functions to convert acid and sample volumes into masses.
* Relocated EMF to [H⁺] conversion functions from `solve` to `convert`, with aliases to maintain backwards compatibility.
* Added `titration` module with new `Potentiometric` class for higher-level manipulation of titration datasets.
* Added plotting functions for `Potentiometric` titration objects into the `plot` module.

* Updated `concentrations.concTotals` and `dissociation.eqConstants` to use [PyCO2SYS](https://github.com/mvdh7/PyCO2SYS) functions to calculate things instead of functions built in to Calkulate.
* Removed all equilibrium constant and concentration functions that were previously internally in Calkulate.
* Added optional total ammonia and hydrogen sulfide concentration inputs. If values are provided, these equilibria will now be taken into account by the `complete` solver.
* Fixed `io.writeDat` function to correctly delimit .dat files with tabs (not spaces).
* Added `simulate.titration` function to directly simulate a titration dataset.
* Updated solver functions to optionally accept `concTotals['C']` as an array.
* Renamed module `vindta` to `datfile`, but added alias to avoid breaking existing code.