Add exception for NEB init with <2 images

[t-young31]

Resolves #266

Raises ValueError with num < 2. A NEB can reasonably be constructed with just 2 images, and is required for the partition method

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Checklist

• The changes include an associated explanation of how/why
• Test pass
• Documentation has been updated
• Changelog has been updated

[codecov]

Codecov Report

Merging #270 (c24414e) into v1.4.0 (7b89cd2) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           v1.4.0     #270      +/-   ##
==========================================
- Coverage   97.26%   97.24%   -0.03%     
==========================================
  Files         195      197       +2     
  Lines       20417    21013     +596     
==========================================
+ Hits        19859    20434     +575     
- Misses        558      579      +21     

Flag	Coverage Δ
unittests	97.24% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
autode/neb/original.py	96.02% <100.00%> (+0.05%)	⬆️
tests/test_neb.py	99.47% <100.00%> (+<0.01%)	⬆️

... and 16 files with indirect coverage changes

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