Fix shell scripts not being added to PATH in software bundle

[djhoese]

Closes #678

As discussed in the related issue, changes in #654 deprecated old and unsupported use of copying/distributing .sh scripts as part of the polar2grid python package installation. When I made that change I didn't realize that I was depending on it as part of the bin/env.sh script.

To any users reading this, we still recommend never sourcing the environment scripts (ex. source $POLAR2GRID_HOME/bin/polar2grid_env.sh, but some users still do. Instead we recommend calling the high-level scripts with an absolute path (ex. POLAR2GRID_HOME/bin/polar2grid.sh).

This PR fixes this problem by softlinking the high-level .sh scripts into the low-level python runtime libexec/python_runtime/bin/ directory. This avoids other things in bin/ being added to your PATH.

In the long run, although not implemented here, the environment scripts should maybe be moved somewhere else (maybe the root directory) and not included in bin/ as they are not scripts to be executed. That's a little too much backwards incompatible for one release so I'm not going to do it here.

[coveralls]

coverage: 85.333%. remained the same
when pulling be3e2e6 on djhoese:bugfix-sh-swbundle-path
into 86b4f28 on ssec:main.