Connection costs not properly factored in or discounted

[hamishbeath]

Describe the bug

When specifying connection costs, the model should calculate the connection costs for initial households as well as new ones. Currently the model misses initial connection costs due to the way the connections_expenditure function is written in the finance.py file. Further, it is more accurate to discount new connections to the year they are connected rather than the start of the increment year.

To Reproduce

Steps to reproduce the behavior:

1. Run a simulation with a positive connection cost and number of households
2. See the incorrect amounts in the appraisal outputs (when compared to calculating on excel)

Expected behaviour

The model should factor in initial connection costs e.g. 100 initial households * connection cost