added docu for heave correction

heave_correction.md

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+# LIMRAD94 - Eurec4a Heave Correction
+
+**Problem:**  
+A Doppler cloud radar measures vertical fall velocities of hydrometeors. Due to the up and down movement of the 
+RV-Meteor (the so called heave), those fall velocities have a systematic error corresponding to the heave rate.  
+The heave rate or heave velocity is heave per second and thus has a unit of m/s. Another component is the roll and 
+pitch induced heave. Since the radar was placed off center of the ship, the roll and pitch movements of the ship also 
+induce a heave motion on the radar. 
+
+**Solution:**  
+Correct the mean Doppler velocity of each chirp by the heave rate, calculated from measurements of the heave by the 
+RV-Meteor. To do this a python function called `heave_correction` is written. Because the correction is applied to each 
+chirp, the function is called in LIMRAD94_to_Cloudnet_v2.py which calculates the radar moments from the measured Doppler 
+spectra.  
+
+## Step by Step
+
+### 1. Calculate heave rate for corresponding day
+
+### 2.