MagFieldElement further follow ATLAS style on inline conventions

e.g

If you have many inline functions, it is usually better to split them out into a separate file, with extension ``.icc'', that is included at the end of the header.

Inline functions can improve the performance of your program; but they also can increase the overall size of the program and thus, in some cases, have the opposite result. It can be hard to know exactly when inlining is appropriate. As a rule of thumb, inline only very simple functions to start with (one or two lines). You can use profiling information to identify other functions that would benefit from inlining.

Use of inlining makes debugging hard and, even worse, can force a complete release rebuild or large scale recompilation if the inline definition needs to be changed.

So move more small inline function in .icc and a couple of large function >20-30 lines to .cxx. From profiler call chains actually these seem no to be actually "inlined" by the compiler most of the times.