Conditionally produce HLT1 line outputs

To allow the SelReports creation algorithm to have optional inputs, HLT1 lines are now responsible for creating the output they want persisted, and the maker is declared as a barrier algorithm. This is an analogous change to that made for HLT2 lines in !716 (merged).

Closes #145 (closed).

Requires LHCb!2954 (merged) and Phys!884 (merged). Required for MooreAnalysis!29 (merged).