Remove thermal conductivity from BPX schema

[ejfdickinson]

spotted by @ikorotkin

The BPX schema includes a field ["Parameterisation"]["Cell"]["Thermal conductivity [W.m-1.K-1]"]. However, this parameter is not defined or used in the corresponding mathematical specification for any model.

Proposed fix:

• remove thermal conductivity from the BPX schema, as it doesn't do anything
• perhaps update the standard document to indicate that thermal parameterisation such as conductivities can constitute user-defined parameters?
• can we support backwards-compatibility for old BPX versions in which this field is allowed?

Recommended to fix after v0.4 release.

[rtimms]

Thanks! On your points:

• Agree
• Agree also
• Yes, let's handle this as a special case in the next version (we can automatically move it to a suitably named user-defined parameter) with a warning that it will be deprecated in the future (say, by v0.6).