Get frame fields by name

[jumale]

At the moment frame contains only list of values, so if you need to get a value from specific field, then you first need to get the frame-definition from reader, then get a field index by name, and then resolve the field value by its index.

We can improve it by introducing function which provides this functionality.
For now I have 3 ideas:

• when we create a frame, we also add there a pointer to *LogDefinition, and then add frame.GetField(name FieldName) (interface{}, error) to frame type (and maybe even GetFieldString(name FieldName) (string, error) and GetFieldInt32(name FieldName) (int32, error))
• (👎single responsibility) instead of passing *LogDefinition to every frame, we could add those methods to the reader, which already has access to *LogDefinition. So it would look like reader.GetFrameField(frame Frame, name FieldName) (interface{}, error)
• or we could just export a stateless function which does the job GetFrameField(frame Frame, name FieldName, def *LogDefinition) (interface{}, error)