Add elementwise multiplication of signals

[mgrub]

Element-wise multiplication (Hadamar product) is a useful operation to enable applications like "windowing" and "gating" of signals in the time and frequency domain. Two new functions are introduced with this PR.

hadamar_product performs element-wise multiplication of two complex vectors of same length with full covariance uncertainty propagation according to the GUM.

window_application applies a real-valued window to a complex-valued signal. It relies on hadamar_product for the calculations.

Tests against slower implementations that use the non-optimized matrix multiplications are provided.