Add "Heat transfer coefficient [W.m-2.K-1]" to BPX schema

[ikorotkin]

It appears in the lumped thermal model (9a) in the BPX doc.

We have "External surface area [m2]", "Volume [m3]", and "Ambient temperature [K]" in the schema, but no "Heat transfer coefficient [W.m-2.K-1]" to make use of the external surface area and the ambient temperature.

[ikorotkin]

I understand it's not a property of the cell itself but it would be convenient to have it as an optional parameter (for the exchange purposes). So, for example, I can fully reproduce my thermal simulation.

[ejfdickinson]

See also #4. I don't think this belongs as a "Cell" property, as you say, but may be useful to have a shortlist of common environmental parameters to support full simulation reproduction.

I would support schema re-organisation to remove temperature from the cell properties, too.

[ikorotkin]

The thermal parameters look inconsistent in BPX schema in general:

• "External surface area [m2]", "Volume [m3]", "Density [kg.m-3]", etc. - optional
• "Ambient temperature [K]" - required (shouldn't be optional?)
• "Initial temperature [K]" - optional (shouldn't be required? A simple NMC example from examples directory does not start in PyBaMM without this parameter)
• "Heat transfer coefficient [W.m-2.K-1]" is defined in the BPX document (lumped thermal model) but does not exist in the schema
• "Thermal conductivity [W.m-1.K-1]" - not used in the lumped thermal model but defined in the schema (related issue Remove thermal conductivity from BPX schema #50)