Search only for boost header-only libraries

Resolves #16 (closed)

NSXTool only uses one Boost library, the header-only interprocess/mapped region, in the ILL Nexus data loader. This change forces Cmake to only find a minimal number of Boost components, namely the header-only headers. This cuts down the number of includes in the Makefile significantly:

With this change, if grep for "boost" in the build directory:

rg boost | wc -l
3402

of these, all but 151 lines are from 3rdparty/xsection.

Without:

rg boost | wc -l
12038