Optimize k_path

[ponweist]

Analyze performance for k_path calculations and do basic optimizations.

[ponweist]

Performance of k_path (16sm case) by removing superfluous call to get_imfghk_list (CPU cycles for master process only):

	Before	Current	Speedup factor
k_path	4e11	3e11	~1.3
get_imfgh_k_list	3.3e11	2.2e11	~1.3

Test parameters:

kpath = T
kpath_task = bands,curv,morb
kpath_num_points = 500
kpath_bands_colour = spin

begin kpoint_path
G  0.0  0.0  0.0  X  0.0  0.5  0.5
X  0.0  0.5  0.5  W  0.25 0.75 0.5
W  0.25 0.75 0.5  K  0.375  0.75  0.375
K  0.375  0.75  0.375 G  0.0  0.0  0.0
G  0.0  0.0  0.0  L  0.5  0.5  0.5
L  0.5  0.5  0.5  U  0.625 0.25 0.625
U  0.625 0.25 0.625  X  0.0  0.5  0.5
L  0.5  0.5  0.5  W  0.25 0.75 0.5
W  0.25 0.75 0.5  G  0.0  0.0  0.0
G  0.0  0.0  0.0  U  0.625 0.25 0.625
U  0.625 0.25 0.625  K  0.375  0.75  0.375
end kpoint_path

kslice = F
berry = F

[ponweist]

After rewrite of get_imfgh_k_list, saving some matrix products which have been evaluated multiple times (CPU cycles for master process only):

	Before	Current	Speedup factor
k_path	4e11	2.7e11	~1.5
get_imfgh_k_list	3.3e11	2e11	~1.6

Comparing with the original code version, computation time (for above parameter settings) is down from 648 to 122 seconds (more than 5 times speedup)!