remove requirement for different scheduling when using or not using presync

[Roland Haas]

In its initial implementation, using PreSync to automatically apply boundary conditions requires changes to user thorns. Namely patterns like this:

schedule Foo_SelectBCs IN MoL_PostStep
{
  LANG: C
} "Select Boundary conditions for variables"
schedule group ApplyBCs as Foo_ApplyBCs in MoL_PostStep AFTER Foo_SelectBCs
{
} "Apply boundary conditions"

needs to be replaced by something like this

schedule Foo_SelectBCs IN Presync_Select
{
  LANG: C
} "Select Boundary conditions for variables"

However it should be possible to remove this need to change schedule.ccl by making the code remember calls to SelectXXXForBC (and possible warn if subsequent calls select different bc for already known variables) and making ApplyBCs an empty group.

[Roland Haas]

This had already been changed in ET_2020_05.