Battery Simulator: Stop charging at the end of the off-peak (fix #200)

[davefiddes]

Only use the desired charge rate for off-peak charging when within the off-peak period. This means we don't hit the desired SOC but don't import more expensive electricity.

Tests:

• Verified with lower 1kW charge rate SOC only gets to ~50%
• Verified with 3kW charge rate, SOC reaches 100% and which is maintained until end of off-peak period and discharge starts