Enforce suite

Nobody has claimed this yet.

The function `set_default_suite` of the ToolRepository is kind of broken. The current behaviour is: the ToolRepository will put the one tool (and I am just changing this while adding MPI support to put all tools of a suite) to the front, and then when looking for a default tool to look from the beginning of the list.

Problem is, if no tool is available for the default suite, this will silently return a completely different tool (e.g. you request 'intel-classic', and might get 'gfortran'). And at some stage (when compiler flags are added to the corresponding compiler, and not as atm to any compiler), you won't even get a warning about invalid flags anymore, since it will just silently use the compiler-specific flags.

So this function should be renamed (`enforce_suite` or `restrict_to_suite`??), and if a special suite has been requested, the `get_default_tool` function should not return a tool that is not part of that suite.

We should still allow the user to explicitly set (different) compilers in a ToolBox, so we cannot remove non-suite tools from the ToolRepository. Suggested implementation: when a suite is requested, the name is validated (check that we have tools of this suite), stored. And when get_default is called and a suite name has been set, restrict this correspondingly.

Start by tracing ToolRepository.set_default_suite and get_default_tool to understand how the default tool is selected and how suite names are represented. Done means a requested suite is validated and prevents fallback to tools outside that suite, while explicitly selected non-suite compilers remain usable.

Written by the indexing model from the issue text.