Updates to be compatible with latest OG-Core

[jdebacker]

To be compatible with OG-Core, which now allows time-varying mortality rates, OG-UK needs some updates:

• Remove references to surv_rate in the default parameters json file, demographics.py, and calibrate.py
• Make rho a nested list in the default parameters json file, and in demographics.py (or to its output in calibrate.py)

[jdebacker]

Update: now just use the ogcore.demographics modules for all demographics