Moving the implemented function of the class reduces
the rapidxml scope to translation unit.

Including
 - usage of static_cast instead of c-style cast,
 - single variable declaration per line,
 - removing const from return types
 - omit unnecessary string to char* conversions,

The used clang-tidy checks are:
google-readability-casting,
readability-const-return-type,
readability-container-size-empty,
readability-delete-null-pointer,
readability-deleted-default,
readability-else-after-return,
readability-function-size,
readability-identifier-naming,
readability-inconsistent-declaration-parameter-name,
readability-isolate-declaration,
readability-misleading-indentation,
readability-misplaced-array-index,
readability-named-parameter,
readability-redundant-*,
readability-simplify-*,
readability-static-accessed-through-instance,
readability-static-definition-in-anonymous-namespace,
readability-string-compare,
readability-uniqueptr-delete-release

Update dependencies.

Most of the current .clang-tidy contained checks:
google-explicit-constructor
google-readability-braces-around-statements
modernize-loop-convert
modernize-use-default-member-init
modernize-use-emplace
modernize-use-using
readability-braces-around-statements
readability-else-after-return
readability-named-parameter
*readability-namespace-comments
*readability-inconsistent-*
readability-redundant-member-init
readability-simplify-boolean-expr
readability-string-compare
*explicit-conversions*

Also use own findElementOrError() algorithm.

Avoid unnecessary initialization.

Constify the remaining variable in the declaration list.

cppcheck message:
Prefer prefix ++/-- operators for non-primitive types.. Prefix ++/--
operators should be preferred for non-primitive types.
Pre-increment/decrement can be more efficient than
post-increment/decrement. Post-increment/decrement usually involves
keeping a copy of the previous value around and adds a little extra
code.

cppcheck message:
class 'Raster' does not have a copy constructor which is recommended since the class contains a pointer to allocated memory.

cppcheck message:
The scope of the variable 'n_intersections' can be reduced.

cppcheck message:
Class 'MinimalBoundingSphere' has a constructor with 1 argument that is not explicit. Such constructors should in general be explicit for type safety reasons. Using the explicit keyword in the constructor means some mistakes when using the class can be avoided.

clang-tidy modernize-use-bool-literals check.