Remove unneeded clone macro impl.
Added divergence operation.
Add functions counting and generating monomials not exceeding a given degree.
Add safety checks for valid fe, oshape, and side arguments.
Fixed warnings related to unused items.
Names clarified in various places to distinguish between space dimensionality (dimension count which is a uint), dimension numbers, and fe side lengths (previously also called dimensions).
Added tests for [num_]mons_of_deg_le functions.
Added separate and expanded tests for vector monomials, including vector_mons_of_deg_le tests.