fill first column with ones
fill first line done
next_element
next_element almost done
check for reverse lattice word conditions
young tableau condition (still some problems)
curr[i+1][j] could be out of the skew tableau (and hence be zero!)
almost there, some bugs around: littlewood_richardson_num doesn't fill the last line
found and fixed bug in young_tableau_condition < instead of <=
fixed bug https://github.com/sciamp/Young_Tableaux/issues/9