revisit newDownload default settings and documentation

[jonathancallahan]

The wrcc_loadYear() function has a default setting of newDownload = NA and documents what this means: NA means reuse it if it is found locally, download otherwise; TRUE means always download even if it is found locally; FALSE means never download even if it isn't found locally.

All of the loading functions should be evaluated for potentially defaulting to newDownload = NA and have their documentation updated to describe what this means.

Some code changes may be needed inside fw13_load().