fix warnings when loading tilesfile

[sbailey]

This PR is a followup to PR #98 which tried to cleanup the case where desimodel.io.load_tiles() is given a tilesfile option that exists both locally and in $DESIMODEL/data/footprint/ ($DESIMODEL wins, albeit with a warning).

The implementation in PR #98 printed incorrect warnings about collisions even when tilesfile was given with a full path and didn't exist in $DESIMODEL, e.g.:

In [1]: !pwd
/global/cscratch1/sd/sjbailey/desi/fiberassign/refactor

In [2]: import desimodel.io

In [3]: tiles = desimodel.io.load_tiles(tilesfile='/global/cscratch1/sd/sjbailey/desi/f
   ...: iberassign/refactor/tiles-lite.fits')
/global/common/software/desi/cori/desiconda/20180709-1.2.6-spec/code/desimodel/master/py/desimodel/io.py:187: UserWarning: File "/global/cscratch1/sd/sjbailey/desi/fiberassign/refactor/tiles-lite.fits" in $DESIMODEL/data is shadowed by a local file.  Choosing $DESIMODEL file.
  warnings.warn(msg)

Despite the warning message saying that it was going to pick the $DESIMODEL file (which doesn't exist), it did correctly load the local file.

This PR updates the logic to avoid the incorrect warning. Summarizing:

• if tilesfile isn't given, use $DESIMODEL/data/footprint/desi-tiles.fits
• if tilesfile includes a path, use that without also checking $DESIMODEL since the full path is specifying a very specific file and we should just use that
• if tilesfile doesn't include a path, it is potentially ambiguous about whether the user intended a local copy or a copy in $DESIMODEL, so check both. If there is a file by that name both locally and in $DESIMODEL/data/footprint/, print a warning and use the $DESIMODEL version (use "./desi-tiles.fits" if you want the local copy to win).

[tskisner]

Looks good, thanks for fixing.