Change plot style functionality to be context based rather than global

[Tom-Kingstone]

Description:

Currently, python_toolkit is (or should be, based on LadybugTools_Toolkit) set up to set the matplotlib style to the bhom.mplstyle. While this does work for setting the plot style for bhom plots, it is set globally and thus also affects modules that are not from bhom.

See https://matplotlib.org/stable/users/explain/customizing.html:
Likely will change to add the mplstyle to the package like is describes so it can be set using plt.style.use("bhom") or similar if a user wishes to set it globally, and then also use local context managers to set the plot style for methods. E.g.
heatmap.py

def heatmap(...):
    with plt.style.context('bhom'):
         #heatmap code here
    return ax

If a user wishes to use a different context for the heatmap, perhaps this could be added as a kwarg style_context which defaults to "bhom"