NR UE & gNB imscope.

A new phy scope based on ImGui and ImPlot. This scope uses a different concurrency model than previous
scopes.

The PHY thread writing the data first checks if the data is ready to be written. If its not, nothing is copied.
The GUI thread reads data if available and marks it ready to write. This makes sure that the PHY threads are not
busy copying data that would never be displayed.

Some of the scopes also have a freeze functionality that further limit the amount of data that needs to be copied
from PHY threads. If a scope is "frozen" it still allows the user to explore the data using plots zoom/pan functions
but doesn't cause PHY threads to perform extra writes on the displayed data.

A compile option was added to enable/disable the scope. Use cmake -DENABLE_IMSCOPE=ON to enable the scope.

Update CXX standard to 17; it is required by some libraries (e.g., dear imgui). The oldest gcc version in
use by a distribution supported is Ubuntu 20, which has gcc 9.4 with c++17 support.

doc/BUILD.md

@@ -42,6 +42,8 @@ Running the  [build_oai](../cmake_targets/build_oai) script also generates some
 
 The build system for OAI uses [cmake](https://cmake.org/) which is a  tool to generate makefiles. The `build_oai` script is a wrapper using `cmake` and `make`/`ninja` to ease the oai build and use. It logs the `cmake` and `ninja`/`make` commands it executes. The file describing how to build the executables from source files is the [CMakeLists.txt](../CMakeLists.txt), it is used as input by cmake to generate the makefiles.
 
+cmake is further extended by using [CPM](https://github.com/cpm-cmake/CPM.cmake). CPM is a cmake script that handles external code dependencies. It is setup to cache downloaded code in `~/.cache/cpm`. While most external dependencies should be handled by system package managers, CPM has the advantage of using any code that is available in a public git repository.
+
 The oai softmodem supports many use cases, and new ones are regularly added. Most of them are accessible using the configuration file or the command line options and continuous effort is done to avoid introducing build options as it makes tests and usage more complicated than run-time options. The following functionalities, originally requiring a specific build are now accessible by configuration or command line options:
 
 - s1, noS1

doc/README.md

@@ -106,7 +106,7 @@ The other SDRs (AW2S, LimeSDR, ...) have no READMEs.
 
 ## Special-purpose libraries
 
-- OAI has a scope based on Xforms, described in [this README](../openair1/PHY/TOOLS/readme.md)
+- OAI has two scopes: one based on Xforms and one based on imgui, described in [this README](../openair1/PHY/TOOLS/readme.md)
 - OAI comes with an integrated [telnet server](../common/utils/telnetsrv/DOC/telnethelp.md) to monitor and control
 - OAI comes with an integrated [web server](../common/utils/websrv/DOC/websrv.md)