User defined attributes

[RTbecard]

Here's the version of Zoon I've been using to work on my stuff. The following changes allow me to work with simulated datasets and detection models.

• User defined attributes are propagated through the entire workflow
• Added functions for users to use to manipulate data.frames without losing attribute data (attributeFunctions.R)
• Added support for workflows with empty rasters and empty occurence modules (useful for simulated datasets where the dataset and covariates are defined via the process modules)

[RTbecard]

I'll try this again... Not sure why there was over 100 changed files in the PR... it should only be 3.