When nlohmann_json is installed in other ways rather
than by using the CMake setup included within the
package, no CMake config file is installed for
nlohmann_json; and so, it is not possible to use
find_package( nlohmann_json ) in the CMake setup of
our package. This commit fixes the build for the case,
for example, when nohmann_json is installed
by simply copying the single-header file into a system
include folder as '/usr/local/include'.

CMake Build Configuration Updates, master branch (2020.03.12.)

See merge request !1

Had to update my feature branch to account for the renaming of the
JSONParser and XMLParser packages/libraries.

renaming XMLParser to GeoModelXMLParser and JSONParser to GeoModelJSONParser (as per Attila's request)

It turns out that GMP is not included in the x86_64-centos7-gcc9-opt
build. So we need to take it from the system in that case instead.
(The Eigen build needs that library...)

It turns out that on SLC6 automake uses the lib/ directory
for libraries by default, while CMake would prefer to use lib64...

It also turned out that the CentOS 7 image didn't have GNU Make
installed in it by default.

In the same style in which GeoModelCore can optionally download
Eigen for itself.

Since the C++ code doesn't actually directly depend on it...

At the same time switched the CI to use LCG_96b, as nlohmann_json
only started appearing in LCG with that version.

Only building its upstream requirements in the first step.

This included moving all the common configuration options into the
project configuration file, exporting all components in the
"GeoModelTools::" namespace, setting up a semi-proper CPack
configuration for the project, and cleaning the configuration
as much as possible.