Input heating rates

[nichollsh]

AGNI should be able to take an array of heating rates and/or fluxes as an additional source of energy transport that can't be calculated self-consistently within the model. This would be loaded from a csv or netcdf file, and be applied to the HR/residual array at solve-time.

Examples of these external heatings include:

• Advective heat transport (e.g in RCAE models)
• Enthalpy from disequilbrium chemistry
• Joule heating from magnetic effects
• Frictional/dissipative heating from tidal effects

[nichollsh]

This will probably need to be solved as a sub-task of Issue #26