Feature/use psychrolib

[azogue]

cc @dmey @shenzha

closes #7

Description

In order to focus on plotting, it has no sense to maintain here a useful collection of psychrometric equations, when now there is a package, PsychroLib with good testing and maintenance in the python orbit, and it also uses a MIT license.

I did this package because of very understandable reasons:

• I was learning python, by making lots of things in the way, and I wanted to plot psychrometric charts to show the air state in my home.
• I searched for some library to do it all, you know, the import antigravity way :), but I didn't find anything usable.
• As I was playing a lot with matplotlib (I came to python from matlab), it was interesting for me to create this package, and as I know some psychrometrics I made the .equations module with good implementations of psy. eqs, documenting the ASHRAE sources in the docstrings.

But it is much better to rely on a multi-platform library like PsychroLib and focus here on plotting. And if something related to psy. calculations can improve, try to collaborate there first.

For the psychrochart project to improve, I think here the things to do need to be only plot-related, like:

• Better handling of plots of a large number of points
• Better plot configuration
• Adapt plot labels, units, etc. to imperial system of units, now that we can set it in PsychroLib

Changes done

• [x] Add psychrolib to requirements and substitute own logic for calls to this library
• [x] Add pyproject.toml file and use poetry to manage the package and travis CI
• [x] Added pre-commit configuration, to enforce that any contribution to the source code maintains some good standards, like the ones made in #8, by a strict combination of flake8 plugins.
• [x] Refactor chart.py into multiple modules:
  • Move PsychroCurve(s) classes definitions to separate module
  • Rename equations to .chartdata and move there all logic for curve data generation
• [x] Adapt tests and do some code cleaning
• [x] Redo internal units for pressure (from kPa to Pa) and enthalpy (from KJ/kg to J/kg) to prepare to switch to IP units
• [x] Adapt plots to use labels, ticks, etc as a function of the selected unit system.

@dmey, any comment that you could suggest would be most welcome.

[azogue]

Merging this now and continuing the repository update in #11