Remove Warnings due to wrong bracketing syntax (was ACTS-386)

Original author Andreas Salzburger @asalzbur

{{/Users/salzburge/Documents/work/acts/acts-sources/acts-core/Core/src/Utilities/BFieldMapUtils.cpp:63:34: warning: suggest braces around initialization of subobject <-Wmissing-braces> Grid_t::index_t indices = {i, j}; ^~~~ { } /Users/salzburge/Documents/work/acts/acts-sources/acts-core/Core/src/Utilities/BFieldMapUtils.cpp:66:49: warning: suggest braces around initialization of subobject <-Wmissing-braces> Grid_t::index_t indicesFirstQuadrant = {i, n}; ^~~~ { } /Users/salzburge/Documents/work/acts/acts-sources/acts-core/Core/src/Utilities/BFieldMapUtils.cpp:58:10: warning: unused variable 'nBinsPhi' <-Wunused-variable> size_t nBinsPhi = 1000; ^ /Users/salzburge/Documents/work/acts/acts-sources/acts-core/Core/src/Utilities/BFieldMapUtils.cpp:161:36: warning: suggest braces around initialization of subobject <-Wmissing-braces> Grid_t::index_t indices = {i, j, k}; ^~~~~~~ { } /Users/salzburge/Documents/work/acts/acts-sources/acts-core/Core/src/Utilities/BFieldMapUtils.cpp:168:49: warning: suggest braces around initialization of subobject <-Wmissing-braces> Grid_t::index_t indicesFirstOctant = {m, n, l}; ^~~~~~~ { } 5 warnings generated.}}