Parse metadata exception handling

[Waqar-ukaea]

PR for #111

[gonuke]

Thanks @Waqar-ukaea - I have a few questions here about the choice of defaults

[gonuke]

This already suggests that the transmission boundary condition should be the default. A surface property is never a material.

Although I wonder if this is a good choice for default if/when other properties get applied to surfaces?

[Waqar-ukaea]

I'm not really certain what the best default for this would be so I'm going to leave it as is for now. Maybe @pshriwise has a better idea of what we should do here.

[pshriwise]

For this I'd recommend a transmission boundary as @gonuke said.

return {PropertyType::BOUNDARY_CONDITION, "transmission"};

[pshriwise]

Although I wonder if this is a good choice for default if/when other properties get applied to surfaces?

Can you expand on what you mean here @gonuke?

[gonuke]

In theory, surfaces could get tagged with metadata for purposes other than boundary conditions, e.g. some variance reduction parameters, etc. In such a case, a default that represents a boundary condition may not be the best default.

[gonuke]

Doesn't the count check above make this already more robust? Can you add tests that fail before this change and pass afterwards?

[gonuke]

Also, could this count approach be used for volume properties as well, instead of the exception?

[Waqar-ukaea]

Agreed on this one too. Added a similar count check for get_volume_property()

[gonuke]

Looking at the Property system in constants.h it seems that

1. There is already a note to formally separate volume properties from surface properties, and
2. There may be room to define some kind of PROPERTY_NOT_FOUND constant

[Waqar-ukaea]

There is probably a wider conversation to be had about how exactly we want to be handling exceptions within XDG. I went with try catch blocks because I'm familiar with them but there probably is a better approach we can take.