Price path optimisation constraints

[Brent-Morrison]

Create functionality so that the optimisation of prices when applied to quantities to equal the revenue requirement can be performed with the following constraints:

1. Single price delta applied to all prices equally, for each of the 5 years.
2. Single price delta applied to all prices for a specified single year.
3. Separate prices for each service type (Water, Sewer, Rural, etc) so that total income for that service equates to a target percentage of the revenue requirement. Prices change equally across. The deviation between differing price paths should be minimised to the extent possible.
4. Set variable tariffs to a specified LRMC price (this is the P5 target price). Set fixed tariffs so that they cover the residual revenue requirement.