Unit test for interpolation

[rdPatmore]

src/pybdynemo_bdy_extr_tm3.py has a self contained interpolation function time_interp(). When modifying this code I made a crude, unautomated version of a unit test by making dummy data and investigating whether the end result was as expected. There was some discussion as to whether we could develop this into a proper unit test.

Tasks/Questions

• Is this a useful test?
• If so, work towards formalising it as a unit test.
• If not, close this issue.