Update boron isotope functions

[rossidae]

Updated boron isotope functions to include:

• Functions for calculations in isotope fractional abundance, supplementing those that were originally there
• Functions for calculations in delta space (i.e. using d11B in place of fractional abundance)
• Full docs for each function
• Nomenclature from ABO3, ABO4 and ABT to AB3, AB4 and ABT (as well as d11B4, d11BT)
• Consistent x_to_y (replacing instance of x_2_y)