As there's currently only one Lua pipeline, we could so far get away
with storing it in the `scripting` directory, giving it a generic name
and registering it manually. However, this approach won't work anymore
once additional Lua pipelines start to be implemented.

To support multiple Lua pipelines, implement a registration mechanism
for scripted pipelines based on PipelineFactory and similar to
PipelineRegistrar. It scans each active module's `pipelines` directory
for Lua pipelines, parses them and generates a registration header that,
when included, registers them with PipelineFactory.

As a result of the above, the test Lua pipeline had to be moved to
`modules/preprocessing/pipelines/` and could be renamed
ResamplingDemoLua.

References #1