[Bug] Inconsistent naming in StrEnums and constructors

[smitchaudhary]

Bug: Currently, if trying to use serial encoding for multivariate functions, it would throw an error because the StrEnum for MultivariateStrategy has SERIES as the string instead of series. This can be fixed by not checking against a literal but only against the MultivariateStrategy.

Cosmetic and consistency related changes:

The naming of some of the StrEnums is inconsistent with the others. Additionally, there was even an inconsistency within a single Enum, namely the MultivariateStrategy.

Should only use StrEnums and not the string literals.