Computes moment and returns the moment object

The *x* coordinate of the centre of mass is defined as *\frac{∑(m_ix_i)}{∑(m_i)}* where *m_i* is the biomass of the *i^{th}* branch and *x_i* is the *x* coordinate of the *i^{th}* branch. *y* and *z* coordinates are similarly computed.
The centre of mass computation excludes branches to be pruned (ie: those whose `toBePruned`

value is set to `TRUE`

).

1 | ```
centreOfMass(object)
``` |

`object` |
A data frame of class |

A vector holding *x*, *y*, *z* coordinates of the centre of mass

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