Thalamo-cortical cell example in BluePyOpt

[lukasgd]

The BluePyOpt integration would benefit from adding support for the thalamo-cortical cell example.

The main feature this example will demonstrate is single cell optimization with non-built-in mechanism catalogues. As discussed in the original PR, non-built-in catalogues can already be used, but there is no example demonstrating this feature yet. The thalamo-cortical example would be a nice use case. Porting it to Arbor will require custom ASC tags in Arbor morphology loaders (#1981).