Plumed 2.8.0 fails metadynamics unit test

[danielhollas]

https://github.com/PHOTOX/ABIN/runs/5409736811?check_suite_focus=true

Metadynamics forces produce non-negligible numerical difference in the unit test. We need to investigate if this is a possible bug in Plumed, or some change in interface or something else. There were apparently some changes in the Fortran interface in 2.8.0, perhaps some of them were not backwards compatible.

ERROR STOP *** Encountered 1 or more failures/errors during testing. ***

Error termination. Backtrace:
Time:         0.032 seconds
  
Failure
 in: 
test_plumed_suite.test_force_metad
  Location: 
[test_plumed.pf:320]
fx(1) changed
AssertEqual failure:
      Expected: <0.99991611203271580>
        Actual: <0.99991594977761633>
    Difference: <-0.16225509946732330E-006> (greater than tolerance of 0.0000000000000000)

[danielhollas]

This is because Plumed started using stretched gaussians, which leads to numerical differences.

See this issue and the implementation. As a quick fix for our test suite, we can set env variable PLUMED_DP2CUTOFF_NOSTRETCH before the linking step.