Change stopping criteria for the SCF cycle

Description

As agreed on the octopus developer's workshop in 2019, the default convergence criteria are updated:

• set ConvRelDens to 1e-6
• set EigensolverTolerance to 1e-7

Moreover, to make sure the convergence criteria are reached, the SCF loop is terminated only if the convergence criteria are fulfilled twice in subsequent iterations.

News snippet

Change stopping criteria for the SCF cycle: tighter convergence thresholds; have to be fulfilled twice in a row

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Closes #166 (closed)