Add missing include for VariableMonitor class

[pelesh]

Description

Closes #331

CC @nkoukpaizan

Proposed changes

Include both, sstream and string in VariableMonitor.hpp.

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.
• N/A The new code is documented.
• The feature branch is rebased with respect to the target branch.
• N/A 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.

[PhilipFackler]

Nice catch