Fixed compilation issue with Swift 5

[andreasjk]

fitscore/Sources/FITS/HDU/HEADER/HDUValue.swift:109:24: Member operator '==' of protocol 'HDUValue' must have at least one argument of type 'Self'

fitscore/Sources/FITS/HDU/HEADER/HDUValue.swift:118:24: Member operator '==' of protocol 'HDUValue' must have at least one argument of type 'Self'

fitscore/Sources/FITS/HDU/HEADER/HDUValue.swift:127:24: Member operator '==' of protocol 'HDUValue' must have at least one argument of type 'Self'

fitscore/Sources/FITS/HDU/HEADER/HDUValue.swift:132:24: Member operator '!=' of protocol 'HDUValue' must have at least one argument of type 'Self'

[tvanpeer]

@andreasjk , just out of curiosity (and impatience), how did you fix this?