Remove -s flag for --schema option in `pdal info`

The -s flag conflicted with --stdin. This patch removes the -s option
for --schema, and adds a test to check the fix.
Merge pull request #281 from gadomski/pdal-info-s-flag

Remove -s flag for --schema option in `pdal info`
Change return type of getField from reference to value.
Merge pull request #283 from abellgithub/getfield

Change return type of getField from reference to value.