Add ubuntu compilation flag to README

See merge request !7

Add a new case to the GXClashPointSystem, related to invalid solids (i.e. boolean solids)

See merge request !6

A solid is considered not correctly defined, so invalid, when it's not possible
to generate points on its surface. In this case gmclash writes an entry in the
json report, writing as coordinates the global coordinates of the center
of the invalid volume. The field distance is filled with a generic non-valid value: -999
These points are visualized in gmex in color folly, under the checkbox InvalidSolid

Fix nlohmann_json dependency on Ubuntu, and the CI pipelines

See merge request !5

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'.

Adapt README for Ubuntu setup

See merge request !4