Make package work w/o data files

[rBatt]

Most of the functions are intended to interact with data files. However, they currently expect the data files to be part of the package, and don't typically require that the data object be passed as an argument to the function. This will cause problems when the data files are not installed along with the package itself.

A new scheme might go like this:

d <- get_data_file('ai') # new function not yet implemented
trawlData_operation(d) # any old trawl data function, will now require that the data.table be passed as argument