Prohibit sloppy syncing

[pelesh]

Description

Prohibit synchronizing memory space if it is already updated. Only memory space that is out of date can be synchronized with the updated space. For example, if host is up to date and device is out of date, then function syncData(DEVICE); will copy data from the host to the device. If the device is up-to-date (i.e. flag gpu_updated_ == true), Re::Solve will return an error. The user is required to keep track of which memory space is current.

Current behavior in Re::Solve is to do nothing if user tries to sync the memory space that is up to date. This provides a convenience for users -- if in doubt whether memory space has been synced, just sync again. This sloppy approach makes certain bugs difficult to track. For example, consider a case where cpu_updated_ == false and gpu_updated_ == true. If user changes vector elements on the host by accessing its raw data and forgets to to mark the host memory as updated, the device memory flag will still show that memory space is the current one. Then, if trying to sync the device with host, function syncData will quietly do nothing. By prohibiting sloppy syncing, this function will return an error alerting user something is wrong.

Closes #282

Proposed changes

• Remove all instances of sloppy syncing inthe code.
• Function syncData returns an error when trying to sync the memory space which is up-to-date.

Checklist

• All tests pass. Code tested on
  • CPU backend
  • CUDA backend
  • HIP backend
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows Re::Solve 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.

Further comments

[shakedregev]

This looks good. I suggest removing "Function call ignored", because this implies that the code otherwise executes normally. Which it doesn't because it errors.