isotopic methods implementation (TODOs)

[AgnieszkaZaba]

• GPU implementation
• comment on total vs light approximation in Bolin number
• molar vs atomic (number of moles)
• mass of dry air calculated every time from density-of-dry-air
• check multiplicity occurrance (dn * multiplicity) like in isotope_methods.py
• Question: do we need molar mass for heavy isotopes; is it possible to define molar Bolin number? (isotope methods)