Pypy does not work well with cpython cimport array.
It is convenient to use memoryviews here
https://cython.readthedocs.io/en/latest/src/userguide/memoryviews.html

* using plumed_massert

* added SIGMA_MIN

* regtest for NLIST and SIGMA_MIN

* check that all walkers read the same file when restarting

* various cleanup

* adjusting inline

* dump state after adding kernel

* added error for too small STATE_WSTRIDE

* better log for adaptive SIGMA

* minor

* check and avoid extremely small adaptive sigmas

* wrong logic

* rescaling height also for adaptive sigma and better handling of extremely small adaptive sigma

@gtribello small fixes

It was a plus sign in the wrong place.  One stinking plus sign, hours of work!