Add Codecov badge to README

Added Codecov badge to README for coverage tracking.

Author: RoryBarnes

README.md

@@ -16,14 +16,18 @@
   <img src="https://github.com/VirtualPlanetaryLaboratory/vspace/actions/workflows/tests.yml/badge.svg">
     <img src="https://img.shields.io/badge/Python-3.9--3.13-orange.svg"></a>
   <a href="https://github.com/VirtualPlanetaryLaboratory/vspace/actions/workflows/pip-install.yml">
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+  <a href="https://codecov.io/github/VirtualPlanetaryLaboratory/vspace">
+  <img src="https://codecov.io/github/VirtualPlanetaryLaboratory/vspace/graph/badge.svg?token=6Z8HVQW8N6">
+</a>
   <br>
   <img src="https://github.com/VirtualPlanetaryLaboratory/vspace/actions/workflows/pip-install.yml/badge.svg">
     <img src="https://img.shields.io/badge/Python-3.6--3.9-orange.svg"></a>
     <img src = "https://img.shields.io/badge/Platforms-Linux_|%20macOS-darkgreen.svg?style=flat">
   </a>
 </p>
 
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 `VSPACE` is a tool to build input files for parameter sweeps with [`VPLanet`](https://github.com/VirtualPlanetaryLaboratory/vplanet).
 With `VSPACE` you can quickly and easily build input files with specific
 parameters with different distributions, such as grids, normal distribution, sines and cosines, and even arbitrary distributions. After generating the trials, use the [`MultiPlanet` package](https://github.com/VirtualPlanetaryLaboratory/multi-planet) to run the simulations