[enhancement] Periodic_function class

[toxa81]

TODO:

• [x] use Spheric_function_set in Periodic_function
• [x] collect pointers from Fortran in a data structure
• [x] set pointers to Fortran function when Periodic_function is created
• [x] @simonpintarelli adjust Python interface, especially for strong type lmax_t
• [x] @toxa81 test parallel runs
• [x] @toxa81 fix and test Exciting code

Summary of the PR:

• use overloaded copy() function to copy opbjects; mdarray uses the same; operator>> is removed
• Periodic_function is now a composition of two objects: Smooth_periodic_function (defined on the FFT grid) and Spheric_function_set (defined for the muffin-tin part)
• Wrappers for external pointers to MT and RG parts of the Periodic_function are introduced
• API functions to set and get Periodic_function are changed to handle the dimensions of the functions correctly
• strong type lmax_t is introduced
• SDDK/type_definition.hpp header is removed
• strong_type.hpp header is introduced
• unused commented code in symmetrize.hpp is removed
• Density::magnetisation() renamed to Density::mag() to be consistent with Density::rho()
• use auto keyword in more places
• copy(), scale() and axpy() are introduced for mixing Periodic_functions
• minor clean-ups

[toxa81]

cscs-ci run default

[toxa81]

cscs-ci run default

[toxa81]

@mtaillefumier sorry for the monstrous PR

[electronic-structure]

squash is the only available option, I will do the merge