Improved support for flags

[hiker]

The new compiler class introduces a separate Flag class, in addition to the existing AddFlags and FlagsConfig.

The Flags and FlagsConfig classes should be combined into one, allowing conditional (AddFlags) and unconditional flags to be used in one object. Additionally to path-specific flags, we need to:

• [ ] Support compiler-specific flags in general (likely move flags from the steps into the compiler class, so we can have different flags for intel and gnu etc)
• [ ] Support compilation modes: fast_debug, release etc - different sets of flags.
• [ ] have conditionals depending on compiler version
• [ ] combination of compiler version and path
• [ ] Need to be able to remove flags (esp. taking OpenMP into account, which might need to work tightly with the compiler object, since the name of the openmp flag is part of the compiler tool)

[hiker]

I'm closing this since the PR/issue number from an unrelated issue turned up in this thread, making it confusing. I have created the same issue as #11.