feat: higher order Taylor expansions (quad/oct)

[nihiL7331]

This PR implements higher order of Taylor expansions (quadrupoles and octupoles).
They act as a source of higher accuracy for those who need it, in cost of performance.
Quadrupoles are toggled as a sensible default (#define PS_ORDER = 2).
Below are some benchmarks showcasing the general difference performance and accuracy-wise between each setting, with THETA=sqrt(12)

ACCURACY TESTS

PS_ORDER = 1
[METRIC] uniform-512: rms_relative_error=0.036429 max_relative_error=0.217696
[METRIC] two-clusters-512: rms_relative_error=0.058618 max_relative_error=0.155663

PS_ORDER = 2
[METRIC] uniform-512: rms_relative_error=0.004189 max_relative_error=0.046053
[METRIC] two-clusters-512: rms_relative_error=0.007404 max_relative_error=0.037780

PS_ORDER = 3
[METRIC] uniform-512: rms_relative_error=0.000971 max_relative_error=0.012762
[METRIC] two-clusters-512: rms_relative_error=0.001346 max_relative_error=0.008965

PERFORMANCE TESTS

PS_ORDER = 1
N,Direct_Time,FMM_Time_ST,FMM_Time_MT
1000,0.000295,0.012052,0.007970
2500,0.001651,0.030599,0.007863
5000,0.006747,0.062310,0.025426
10000,0.026680,0.138287,0.046062
15000,0.061499,0.220854,0.056385
20000,0.108018,0.292596,0.074394
25000,0.173939,0.381255,0.098638
50000,0.702443,0.797964,0.197827
75000,1.653841,1.267395,0.306517
100000,3.227759,1.785016,0.424795
150000,7.673903,2.694467,0.645317
200000,12.886704,3.581957,0.959547

PS_ORDER = 2
N,Direct_Time,FMM_Time_ST,FMM_Time_MT
1000,0.000294,0.013998,0.012526
2500,0.001699,0.038598,0.008462
5000,0.006940,0.089117,0.031664
10000,0.029166,0.194092,0.045111
15000,0.066810,0.298451,0.069497
20000,0.122342,0.411555,0.098001
25000,0.193367,0.540451,0.128979
50000,0.774554,1.167462,0.265204
75000,1.777199,1.820340,0.422292
100000,3.239737,2.480748,0.582213
150000,7.637159,3.806646,0.917729
200000,13.223941,5.074902,1.274119

PS_ORDER = 3
N,Direct_Time,FMM_Time_ST,FMM_Time_MT
1000,0.000257,0.017439,0.018197
2500,0.001653,0.054577,0.021085
5000,0.006793,0.130414,0.034317
10000,0.028812,0.290678,0.076779
15000,0.067244,0.456550,0.119921
20000,0.120827,0.617777,0.164437
25000,0.191577,0.803649,0.206227
50000,0.776518,1.754386,0.460201
75000,1.777746,2.771423,0.715856
100000,3.227438,3.826405,0.991993
150000,7.677042,5.730357,1.544024
200000,13.468767,7.907193,2.099430

NOTE: bad performance is caused by high theta value, a sensible default for real-time simulations and PS_ORDER>1 is THETA=1.5F.