a string value, which has to be one of 'triangular', 'uniform', 'epanechnikov', 'quartic', 'gaussian'
adaptive_bandwidth
(optional) TRUE (default) or FALSE: TRUE use adaptive bandwidth calculated using distance of k-nearest neithbors,
FALSE use max distance of all observation to their k-nearest neighbors
use_kernel_diagonals
(optional) FALSE (default) or TRUE, apply kernel on the diagonal of weights matrix
power
(optional) The power (or exponent) of a number says how many times to use the number in a multiplication.
is_inverse
(optional) FALSE (default) or TRUE, apply inverse on distance value
is_arc
(optional) FALSE (default) or TRUE, compute arc distance between two observations
is_mile
(optional) TRUE (default) or FALSE, convert distance unit from mile to km.