Add tests for coverage

[patrick-reinhard]

• Adding tests, increasing coverage of welly dir to 73%
• PEP8 formatting
• Small fixes for Curve methods, passing through keyword arguments.
• Two fixes to make welly compatible with python versions 3.8, 3.9, 3.10
  • pd.set_option('display.precision') and pd.set_option('precision')
  • ax.plot() not working when passing a df and an np.array, transform df to np.array before passing to plot function

[kwinkunks]

Nice - thank you.

NB You have tests themselves and some PyCharm stuff in there. I just added a .coveragerc file which solves this issue for me; probably depends how you're running tests.