feat: add method to access fele in pppm/dplr (#4452)

- I would like to modify the fele vector in pppm/dplr (e.g., by a fix
plugin), which would be used to calculate the long-range interaction in
fix dplr.
- Therefore, I would like to add a new method to access the editable
fele. This method would not change the current workflows of the
lammps-dplr interface.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Introduced a new method to access a vector of doubles, enhancing the
class interface for better usability. This allows for external
modification of the vector.

- **Bug Fixes**
	- No bug fixes included in this release.

- **Documentation**
	- Updated documentation to reflect the addition of the new method.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: HanswithCMY <167949665+HanswithCMY@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Han Wang <92130845+wanghan-iapcm@users.noreply.github.com>

source/lmp/pppm_dplr.h

@@ -28,6 +28,7 @@ class PPPMDPLR : public PPPM {
   ~PPPMDPLR() override {};
   void init() override;
   const std::vector<double> &get_fele() const { return fele; };
+  std::vector<double> &get_fele() { return fele; }
 
  protected:
   void compute(int, int) override;