update to roxygen 4.0.2
remove unused flip argument
Give dpscan a db option to match nat::nlscan
refactor dpscan to wrap nlscan

* need to handle the db argument in the same way as plot3dfc
doc fixes and examples
doc: fix link to nlscan
fixes in hclustfc docs
reference nhclust in hclustfc docs

* fixes #35
switch from httr to RCurl for header check
test for error if fc_download_data bad url
Merge branch 'fix/fc_download_data_head'

* closes #34
* nb any existing cached header files will effectively be ignored
* fix/fc_download_data_head:
  test for error if fc_download_data bad url
  switch from httr to RCurl for header check
only save cached version of headers after download
teach download.file.wcheck to accept a destdir arg

* i.e. specify download location rather than absolutely specify file
refactor download.file.wcheck to include etag check

* simplfiying fc_download_data and enabling it to check for updates in both members
  of a bigmat or ff pair
export and document download.file.wcheck

* consider adding this to nat.utils