[RFC] Replace ParameterSet::existAs with ParameterSet::get*Parameter<std::optional<...>>

[Dr15Jones]

We could use edm::ParameterSet::get*Parameter<std::optional<>> as a replacement for existsAs. The idea would be the fillDescriptions would add something special internal to the edm::ParameterSet for an optional that does not contain a default. Then only if that optional was declared in the fillDescriptions does the edm::ParameterSet::get*Parameter<std::optional<>> not throw if called. This fixes a problem with existsAs if one happens to have a typo using the wrong class type or wrong parameter name.

[Dr15Jones]

assign core

[cmsbuild]

New categories assigned: core

@Dr15Jones,@makortel,@smuzaffar you have been requested to review this Pull request/Issue and eventually sign? Thanks

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A new Issue was created by @Dr15Jones.

@Dr15Jones, @antoniovilela, @makortel, @mandrenguyen, @rappoccio, @sextonkennedy, @smuzaffar can you please review it and eventually sign/assign? Thanks.

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