Fixed Step IDA

[Steven-Roberts]

Description

Adds option to run IDA with fixed time steps

Closes #326

Proposed changes

Adds dt argument to IDA constructor. If nonzero, that fixed time step will be used, otherwise the time step is adaptive.

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.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

Further comments

Any preference on passing dt via constructor versus getter/setter @pelesh? Whatever we decide for dt I can do for tolerances and max steps too.