Placing libs and execs in correct dirs

[paulromano]

Moving issue 2 from gitlab originally submitted by @RonRahaman:

Right now, the OpenMC targets (openmc, libopenmc, etc.) are built in reasonable directories (./lib/ and ./bin/). However, the Nek5000 and STREAM targets are not put in reasonable directories.

Post-build, the targets will be placed like this, relative to the build directory:

• ./bin/openmc
• ./lib/libfaddeeva.a
• ./lib/libopenmc.so
• ./lib/libpugixml.a
• ./lib/libpugixml_fortran.a
• ./src/nek5000/libnek5000.a
• ./src/nek5000/nek5000
• ./libstream.a
• ./stream

I suggest we build:

• libstream and libnek5000 in ./lib/
• nek5000 and stream in ./bin/