Set ulimit of core to -1 enable core dumps for file-based core dumping.
Note that it is not necessary for e.g., systemd-coredumpd, but put it
for completeness.

- Print problems, e.g., segfault
- Handle signals and zombie processes properly

Some CI users (OAI developers) expressed that the warning of the `docker
volume prune` before starting RFsim is confusing. To avoid the warning in
the HTML, force the removal; if the volume does not exist, no warning is
emitted.

To still catch possible problems, do not do this in the `terminate.xml`
so we might still see if there are other problems.

The original UL max MCS of 9 was chosen for performance reasons. In the
meantime, machines became faster, and the ULSCH decoding faster. Improve
UL max MCS to be 28 by default to use the complete range of possible
MCS.

Doing only 5 LDPC iterations by default leads to a poor reception
performance, and correspondingly many retransmissions. Increase to 8 as
the new default to improve user experience.

move command line sample offset and 3/4 sampling option at top level, as it is common option for 4G,5G xNB and UE

add LOG_DDUMP() to dump a buffer in the regular tracing filter system (like PHY, MAC, ...), use it and do simple cleaning (better format)

- improves code readability and maintainability

- centralized channel simulation documentation in channel_simulation.md
- removed redundant documentation repeated in multipled .md files
- updated links
- removed obsolete information
- introduced mermaid flowchart for rfsim
- added missing information about available options and usage
- deleted:    ../../../openair1/SIMULATION/TOOLS/DOC/devusage.md
- deleted:    ../../../openair1/SIMULATION/TOOLS/DOC/rtusage.md
- deleted:    ../../../openair1/SIMULATION/TOOLS/DOC/arch.md

related to issue #739

Ensure that the UE has an IP address before continuing the test.
Otherwise, it might happen we ping before the UE is connected.

Do not do this in do-ra and phytest, as they use --noS1, which implies
statically assigned IP address, so IP address check always succeeds(!).

Make RFsim start up faster by starting core and RAN start at the same
time. This speeds up because we do not wait for "healthy" containers
serially, but parallely.

For the RAN, it is safe to start gNB and UE at the same time in SA mode
(as the UE will only attach once the radio, and hence the full gNB, is
ready).

We do not start in parallel in do-ra and phytest, as in these modes the
gNB first has to write a UE-specific configuration file. If we start
both gNB and UE at the same time, the UE might try to read the file
before it is written, leading to a failure.

- comments to improve readability of the code
- Clang formatting of E1 / RRC (partial)

- new folder dedicated to E1AP docs
- new file to document E1AP procedures