added support for lambda on two mode (no ticket)

[geishm-ansto]

Description of work.
The previous pelican loader could not handle data captured in lambda on two mode (halves the wavelength), this adds this capability.

To test:
The loader test code has been updated.

Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

Does everything look good? Mark the review as Approve. A member of @mantidproject/gatekeepers will take care of it.

[Matthew-Andrew]

Code looks good and has appropriate tests, also worked for me locally.