Update ngen Docker image to build t-route master branch

[robertbartel]

Switch to building/installing t-route master instead of t-route ngen branch. Very similar to changes from #318, but with some modifications based on initial review and evolution of the repo since.

Additions

• Several small build system improvements including:
  • using ${BUILD_PARALLEL_JOBS} instead of $(nproc)
  • an improved way to optionally pass a pre-downloaded boost tarball to the image creation process, without reducing Docker build cache utilization
  • remove of the requirement for t-route repo/branch values to be explicitly provided in the .env file

Removals

• Does not work out of the box to use legacy t-route (e.g. ngen branch), which might limit capability with some older hydrofabric datasets.

Changes

• Now compiles and builds master t-route by default (instead of ngen branch)
• Several Python elements are now installed in the global (root) site. This enables use-cases where a user other than mpi(1000) is used, e.g. using docker run --user=$(uid -u), so now the routing modules are available to that user
  • These changes were carried over from Change routing build to t-route master branch #318; they were tested there but have not been subsequently retest as of this writing

[aaraney]

Looks good to me!