Remove conversion from string to DetectorSizeConstants in DetectorParams

[DominicOram]

We used to need to be able to accept a string as DetectorSizeConstants as it was a parameter from GDA but this is now used as a constant in Hyperion so we can remove the need to derive it from a string.

Acceptance Criteria

• Remove ability to specify DetectorSizeConstants as a string
• DetectorParams still converts to json and back ok