MFSFR2 assign_layers hardcoded output path

[wpbonelli]

Currently ModflowSfr2.assign_layers() writes to sfr_botm_conflicts.chk in the current working directory rather than in the model workspace:

https://github.com/modflowpy/flopy/blob/1757470fe0dd28ed6c68c95950de877bd0b4a94b/flopy/modflow/mfsfr2.py#L1133

The check() method on the same class writes to the model workspace by comparison:

https://github.com/modflowpy/flopy/blob/1757470fe0dd28ed6c68c95950de877bd0b4a94b/flopy/modflow/mfsfr2.py#L1096

Should assign_layers do so as well?