time as difftime breaks calculate_kinematics()

[roaldarbol]

I need to come up with some concrete decisions about time. Should it be:

1. A simple numeric (for seconds) - makes further calculations easier
2. Difftime - is automatically generated from datetimes when subtracting them
3. datetime - when we simply keep the absolute timings.

I think that we should probably just always convert difftime to numeric if we do so, and document that. Should include tests as well! Make the same decisions for respirometr for compatibility.

[Gerrry27]

If you convert "time" to numeric it might be useful to add to the code a line copying "time" to another column ("Date_time"?) before it is converted. In this way, the time info is still always understandable and easy to read.

[roaldarbol]

Good point. Maybe we could always use "time" as the time since the beginning of the data, and have a separate column reserved for absolute/datetime, something like "datetime"/"dt"/"time_absolute"?

[Gerrry27]

Sounds good to me! Maybe "datetime" is the easiest wording to comprehend what it is about.

[roaldarbol]

@Gerrry27 Could do - I'm not wholly convinced though; if you then filter out the e.g. the first 3 minutes, then it will be wrong. Can you add the comment to #30 instead, as it's not really related to the time issue here. :-)