Reconfigure DASSH installation process

Right now DASSH is installed into its own conda environment unnecessarily. The PyDASSH.py module then can either run dassh from python or bash by calling the dassh_run.sh script. It is not necessary to install DASSH into its own separate environment so this change installs it directly into the Workbench environment. Because it gets installed there, there doesn't need to be the dassh_run.sh script anymore to invoke the separate env. Furthermore, DASSH must be installed into the runtime environment being used (whether that is Workbench or the user's own environment), and therefore the dassh python module must always be available. As such, I removed the option to run from bash for simplicity. The changes to how the dassh python module is found and used mirrors how it is done for OpenMC in PyMCSim.py. Also in this MR is linking the DASSH script to a specific version of DASSH that I just made a release of on Github. This tagged version has a fix for the encoding error I am seeing in !221 (merged).

This is a start to some reconfiguring of the process of installing dependencies as described in #83 (closed).