Fix interpolation of nans in decoding position

[samuelbray32]

Description

Resolves #1032

• When upsampling data in PositionGroup preserve nan position values from source
• In ClusterlessDecodingV1 and SortedSpikesDecodingV1
  • Set is_training = False for nan valued position timepoints

Checklist:

• no This PR should be accompanied by a release: (yes/no/unsure)
• n/a If release, I have updated the CITATION.cff
• no This PR makes edits to table definitions: (yes/no)
• n/a If table edits, I have included an alter snippet for release notes.
• n/a If this PR makes changes to position, I ran the relevant tests locally.
• I have updated the CHANGELOG.md with PR number and description.
• n/a I have added/edited docs/notebooks to reflect the changes