Started rewriting tides page

So far, the section on gravity has been written.
Rewrote Tides page

Lots of new stuff here. Still needs proof-reading and some of the figures need checking since I don't have an internet connection at the moment.
First edit of the rewritten Tides page

Fixed a few typos and reworded a few sentences.
Further edits to the Tides page

Improved explanation of tidal ranges, improved spring and neap tide diagrams and fixed a couple more typos.
Final edit to the Tides page

Corrected a couple of numbers and reworded one or two sentences
Merge branch 'tides-rewrite'