495/mem leak lru cache

[kasbaker]

Bugfix for memory leak in EphysNWBData caused by @lru_cache decorator.

Overview:

Importing lru_cache from methodtools instead of functools fixed the leak.

Addresses:

#495

Type of Fix:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected)
• Documentation Change

Changes:

• Added bug fix description to version 1.0.3 section of CHANGELOG.md
• Changed import package on line 3 of ephys_nwb_data.py from functools to methodtools
• Updated version.txt from 1.0.2 to 1.0.3
• Added methodtools to requirements.txt

Validation:

See #495 (comment)

Configuration details:

• OS & version: Centos 7
• Python version 3.6.12
• AllenSDK version 2.5.0

Checklist

• My code follows
  Allen Institute Contribution Guidelines
• My code is unit tested and does not decrease test coverage
• I have performed a self review of my own code
• My code is well-documented, and the docstrings conform to
  Numpy Standards
• I have updated the documentation of the repository where
  appropriate
• The header on my commit includes the issue number
• My code passes all tests
• I have updated the CHANGELOG.md with the description of changes understandable by end users