Update reservoir precip accumulation implementation

[AnnaKwa]

Short description of why the PR is needed and how it satisfies those requirements, in sentence form.

(Delete unused sections)
Added public API:

• symbol (e.g. vcm.my_function) or script and optional description of changes or why they are needed
• Can group multiple related symbols on a single bullet

Refactored public API:

• Bulleted list of removed or refactored symbols, such as changes to name, type, behavior, argument, etc. Be cautious about doing these and discuss with team more broadly.

Significant internal changes:

• Bulleted list of changes to non-public API

Requirement changes:

• Bulleted list, if relevant, of any changes to setup.py, requirement.txt, environment.yml, etc

• Ran make lock_deps/lock_pip following these instructions

• Add PR review with license info for any additions to constraints.txt
  (example)

• Tests added

Resolves # [JIRA-TAG]

Coverage reports (updated automatically):