Short PR focused on optimisations I have made after reviewing the results from profiling with Intel VTune hotspots analysis which provides some metrics for hot functions/paths within the code.

Changelist

• Created a new function inline CoordTransform::cart_to_polar_xy() which handles conversion to polar coordinates returning only (R,Z) as the calculation of the angle phi relies on the expensive atan2 function. Existing full cart_to_polar() function which returns (R,Z,PHI) still exists and is used in some places where the angle phi is required such as Cartesian b-field calculation
• New function inline CoordTransform::cart_to_polar_phi() to only return phi and not (R,Z)
• Changed a lot of older functions that passed by value to now pass by const reference instead
• Added a new function EquilData::get_psi(R,Z) to get the value of psi at a particular polar coordinate (R,Z) instead of having to do a full polar_to_flux transformation as was done previously
• Cleaned up function definitions under CoordTransform namespace
• Cleaned up various other functions to not waste time declaring 'std::vectors' when they don't need to in an attempt to speedup new and free times which were identified as a top hotspot in VTune analysis