Bug fixes, more units, improvements to printing

• Fixed output of exponents with decimals. Calculations were correct, but the display of the equations was incorrect.
• The & symbol can now be used in equation descriptions and comments.
• Added units: ton, tonne, tonf, tonnef
• Worked on ipycalc exporter. Trying to get it to automatically remove cells with tags 'hide_input' and 'hide_cell'.