Limit the reading of CSVs in seed directory to dbt seed

[chrismeier]

When using dbt commands beyond dbt seed (i.e. dbt run), the parse_seed_file function in parser.py loads the CSV file unnecessarily. In the event that there are larger CSVs in the seed directory, this load time can take a while.

To limit the loading of the CSVs, the following section of parse_seed_file should only run during dbt seed.

 try:
        table = dbt.clients.agate_helper.from_csv(abspath)
    except ValueError as e:
        dbt.exceptions.raise_compiler_error(str(e), node)
    table.original_abspath = abspath
    node['agate_table'] = table

The original Slack conversation that this issue came up in is here.

[beckjake]

Fixed in #1046

[chrismeier]

This is super exciting @beckjake! Thanks for putting a together a fix -- looking forward to testing it out :)