OPES_METAD cleanup (#672)

* 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

* allow for conversion from kernels to state

* added OPES_METAD tutorial

* small fixes

* better metad reweighting disclaimer

* adjust kbt and some other minor things

* typo

* cleanup