Remove strong dependency on mlegp

[bgctw]

The Lasslop daytime partitioning of GPP currently relies on smoothing parameters using a Gaussian Process regression using package mlegp. However, mlegp is being archived from CRAN threatening to also removing REddyProc from CRAN.
Hopefully mlegp authors will fix this issue. Otherwise
In the longer-time do the smoothing using some other method.

In the short-time we need to replace the strong dependency to mlegp by a weaker dependency (in Suggests) and require the user to install a maybe old version of mlegp by hand.

email from CRAN maintainers: "
We have asked for corrections to the errors showing at

https://www.stats.ox.ac.uk/pub/bdr/donttest/FCPS.out
https://www.stats.ox.ac.uk/pub/bdr/donttest/penalizedSVM.out

with no response from the mleqp and densityClust maintainers where the errors originate. That for mlegp even shous in Rdev CMD check --as-cran:

• checking R code for possible problems ... NOTE
  Found if() conditions comparing class() to string:
  File ‘mlegp/R/mlegp.R’: if (class(t) == "try-error") ...
  Use inherits() (or maybe is()) instead.

Thus those packages are now scheduled for archival on 2022-03-20, and archiving them will necessitate also archiving their strong reverse dependencies

REddyProc REddyProcNCDF TAG c060 monocle penalizedSVM

if they still require one of these packages by that date.
"