Virtual product formula transformation

[uchchwhash]

Reason for this pull request

Add a useful transformation formula that supports Python's expression language
to do band maths in virtual products.

Proposed changes

• Add formula transformation
• Add simple test
• Update documentation to mention the transformation
• Add lark-parser as a dependency

[codecov]

Codecov Report

Merging #699 into develop will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #699      +/-   ##
===========================================
+ Coverage    88.51%   88.56%   +0.05%     
===========================================
  Files          105      105              
  Lines         9261     9302      +41     
===========================================
+ Hits          8197     8238      +41     
  Misses        1064     1064

Impacted Files	Coverage Δ
datacube/virtual/__init__.py	80% <100%> (ø)	⬆️
datacube/virtual/transformations.py	87.41% <100%> (+4.68%)	⬆️

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[coveralls]

Coverage increased (+0.05%) to 88.519% when pulling 65b3a04 on virtual-product-formula-transformation into db97e2c on develop.

[andrewdhicks]

This is a very clean implementation! Nice work!