Update code

[MatthewDaws]

Time marches on, and APIs change. The build is currently failing.

• Caused by the Python 3.6 build, specifically:

  >   times = [np.datetime64("2017-01-01") + np.timedelta64(days=i) for i in range(10)]
  E   TypeError: 'days' is an invalid keyword argument for this function
  

• See https://numpy.org/doc/stable/reference/arrays.datetime.html

Also a whole load of warnings:

sepp\grid_nonparam.py:17
  C:\Users\matth\Documents\GitHub\SEPP\sepp\grid_nonparam.py:17: DeprecationWarning: invalid escape sequence \s
    """Non-parametric based triggering kernel

Should also add Python 3.7 and 3.8 and 3.9? But need to get open_cp up to scratch, which is more work.