Upgrade dHdt

[smitdave]

• [ ] What is the dts version of dHdt?
• [ ] Add and test setup for human demography & aging
• [ ] Check that ramp.xde includes the basic models
• [ ] Add a realistic human aging module, with utilities that work with age-structured human data
• [ ] Add steady_state_XH

[smitdave]

• Add "birth_stratum" vector

[smitdave]

• add constant birth
• add constant per-capita birth
• add constant per-capita death