WIP: Task class

[nakul680]

Closes Issues

Closes #
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Description

• Implements TaskParams and Task classes
• Implement way to lookup simple keywords from strings

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Release Notes

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Added

Changed

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[timmyte]

@nakul680
Did you also take into account, that we substitute some letters/characters in the original names and that both key and value could be used to lookup the respective enum member, e.g.

G6_311GSTARSTAR = SimpleKeyword("6-311g**")

Searching for G6_311GSTARSTAR or 6-311g** should both yield the same enum member.

[nakul680]

• Changed TaskParams to Settings, which have child classes 'TaskSettingsandMethodSettings`.
• Task class renamed to SimpleTask, added method_settings and task_settings parameters.
• validate() function in Settings class has been repurposed to contain cross-validation logic (which is not implemented yet) , as validate_field() function already handles all validation
• Added documentation
• created new file method_settings.py I think all child classes of MethodSettings i.e the 4 major groupings like DftSettings, WftSettings etc. should live here since they are not task-dependant.

[nakul680]

Latest changes:

• Introduction of new groupings like ScfConvergence , ScfThreshold etc. These are now the base classes holding keyword of this type, and the larger grouping Scf will inherit from these classes.
• Fleshed out DftSettings class , added cross validation in case of specific methods, and added logic to look for dispersion correction keywords.
• added strict flag to avoid overwriting existing working directories.
• solvent, solvation_model are now optional keywords.
• added aliasing for simple keywords to allow for a better API