Display prediction / observation in absolute terms

Rather than showing absolute tide level and relative surge prediction /
observation, show everything in terms of actual sea level heights.

This is the result of feedback from the pilots.

Also improve(?) the column headings.

[#82936070]
Move rounding logic

The TideLevelRow should be responsible for formatting the floats its
given - rather than being passed formatted strings. This will be
required for the up/down indicator arrows.
Up/down indicators for prediction & observations

To show at a glance how the prediction/observation compared with the
harmonic tide prediction.

https://www.pivotaltracker.com/story/show/82936070 [Delivers #82936070]
Merge pull request #47 from sealevelresearch/rejig-tide-levels-table

Improved predictions/observations table