use sparse polynomials for generating functions

[mantepse]

Currently, the findstat interfaces uses dense polynomials to store the generating functions for a statistic. However, some generating functions are extremely sparse, for example https://www.findstat.org/StatisticsDatabase/St000001/ has

q^292864 + q^68640 + 2*q^64064 + 2*q^48048 + 4*q^21450 + q^16016 + 4*q^15015 + 2*q^12870 + 2*q^11583 + q^8580 + 4*q^7700 + 8*q^5775 + 4*q^4455 + 4*q^4158 + 2*q^3520 + q^3080 + 4*q^2970 + q^2640 + 16*q^2310 + 2*q^2145 + 2*q^2112 + 4*q^1540 + 8*q^1320 + 4*q^1188 + 4*q^1155 + 4*q^1092 + 4*q^1017 + 4*q^990 + 2*q^825 + 16*q^768 + 2*q^660 + 8*q^567 + 2*q^552 + 8*q^525 + 4*q^471 + 2*q^462 + 8*q^450 + 3*q^448 + 4*q^432 + 8*q^384 + 2*q^309 + 4*q^300 + 6*q^288 + 2*q^282 + 4*q^252 + q^244 + 24*q^216 + 4*q^210 + 12*q^189 + 16*q^168 + 12*q^162 + 4*q^146 + 2*q^132 + 4*q^126 + 4*q^120 + 2*q^112 + 4*q^99 + 4*q^98 + 4*q^92 + 12*q^90 + 6*q^84 + q^80 + 4*q^77 + 25*q^70 + 14*q^64 + 14*q^56 + 8*q^55 + 8*q^49 + 8*q^42 + 4*q^40 + 2*q^36 + 32*q^35 + 4*q^30 + 6*q^29 + 4*q^28 + 8*q^26 + 16*q^21 + 8*q^20 + 12*q^19 + 19*q^16 + 14*q^15 + 18*q^14 + 10*q^11 + 14*q^10 + 16*q^9 + 10*q^8 + 14*q^6 + 30*q^5 + 20*q^4 + 32*q^3 + 19*q^2 + 26*q}

Storing this in a dense polynomial is not a good idea.

[nadialafreniere]

LGTM. Thanks!

[github-actions]

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