Modernize FSRNavigator

• avoid making spurious string copies
• prefer inheriting constructors

mentioned in merge request !1263 (merged)

• [2018-04-22 00:10] Validation started with lhcb-lcg-dev3#502
• [2018-04-22 00:10] Validation started with lhcb-gaudi-head-py3#131
• [2018-04-22 00:10] Validation started with lhcb-lcg-dev4#502
• [2018-04-22 00:12] Validation started with lhcb-clang-test#928
• [2018-04-22 00:13] Validation started with lhcb-tdr-test#127
• [2018-04-22 00:16] Validation started with lhcb-gaudi-head#1843
• [2018-04-22 00:16] Validation started with lhcb-head#1826

merged

mentioned in commit 7d36f805