Fix IDA initial condition tol for fixed steps

[Steven-Roberts]

Description

Fixes a bug where tolerances were too loose when fixed step IDA solved for consistent initial conditions. Adaptive IDA was unaffected.

Proposed changes

Adds missing IDASetNonlinConvCoefIC call.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• The CHANGELOG.md has been updated to reflect the changes. If this is a minor PR that is part of a larger fix already included in the file, state so.

Further comments

[superwhiskers]

i'll merge this once this passes ci