KoRiF

add global variable to configure datasets directory

3

this allows to load CIFAR10 & CIFAR100 datasets from the arbitrary location by default, directory initialized as empty string, so this should not affects on the existing logic usage: `...

generalize value of TRestContentType type parameter in the RESTRequest.AddBody() call using implicit string value for this parameter leads to compile-time error on Delphi 10.4 Community Edition