test to show read.neurons ignores args for zip file

* encapsulates https://github.com/jefferis/nat/issues/193
Fix: read.neurons processes all args for zip input

* closes https://github.com/jefferis/nat/issues/193
Fix writing neurons to zip file with subdirs etc
fix write.neurons when writing to zip with rel path

* the file was being written relative to the temp dir not the
  original working directory.
* test for this as well
* really closes https://github.com/jefferis/nat/issues/191
Give write.neurons a Force argument (for zip output)

* This will check for an existing zip file and ensure that files are not
  added to an existing zip file on disk, which is the default behaviour for zip()
Merge branch 'fix/read-write.neurons-zip'

* fix/read-write.neurons-zip:
  Give write.neurons a Force argument (for zip output)
  fix write.neurons when writing to zip with rel path
  Fix writing neurons to zip file with subdirs etc
  Fix: read.neurons processes all args for zip input
  test to show read.neurons ignores args for zip file