Function to linearly interpolate state mutable sequences

[gawebb-dstl]

The function will take a StateMutableSequence and a times: List[datetime.datetime]. The function will create a new StateMutableSequence with states only at the times specified by times.

This time with a clean git history. See #865 for messy history

This provides a step for fixing #364

[gawebb-dstl]

I've looked at 4 options (see code). All 4 options run function correctly. Option 4 is the quickest* so will be taken forward

Output when running the file:

Option 1 took 3.9378470000000005
Option 2 took 79.7136048
Option 3 took 1.762432000000004
Option 4 took 1.326851700000006
Option 1 worked
Option 2 worked
Option 3 worked
Option 4 worked

*Option 3 is quicker if all of the states need to be produced by interpolation