Assimp3 install and POSIX standard compatibility

(Note: Removing bashisms makes the scripts POSIX compliant which "ensures" compatibility with anything a platform symlinks */bin/sh* to. )

I started fixing the *assimp2* uninstall and *assimp3* install steps and ended up removing all bashisms from ~~allocore's install dependencies~~ every shell script.

Most importantly, I modified *run.sh* and *debug.sh* .

A few points:
1. The fixes to the *mingw* stanzas need to be tested by someone with that platform.
2. ~~I need to go over the other install dependencies scripts... and all *"sh"* scripts for that matter.~~
3. We ensure that *assimp3* is used because compilation completes, but is wrong if there is a *assimp2* and *assimp3* mismatch between headers and libraries.

Canonical's [guide for porting from bash to dash](https://wiki.ubuntu.com/DashAsBinSh) was useful.
Any reason not to rm the packages any more?