geospace-code/h5fortran: Read character improvement, install and build type defaults

read character: automatically truncate to c_null_char if present this avoids the user having to do this themself. Before this, if the user string variable was longer than the disk string variable the remainder of the user variable could be filled with garbage non-blank characters.

default install to top-level project binary dir this makes the default install directory visible even when used from FetchContent

use CMake generator expressions to simplify / clarify logic