Allow longer units grouping

[bart1]

For prettier printing longer units groupings would be useful. Currently the limit for unit grouping string is only one character (units_options) with longer unit for example math parsing in markdown document is possible

The following document exemplifies this (spaces inserted for code blocks):

---
title: "units example"
author: "Bart"
date: "3/10/2021"
output:
  pdf_document: default
---

` ``{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
require(units)
` ``

## old

`r set_units(1,'m')`

`r format(set_units(1,'m'))`

`r format(set_units(1,'m*m'))`

`r format(set_units(1,'1/(m*m)'))`

## modify group

` ``{r}
opt<-getNamespace('units')$.units_options
opt$.units.group <- c('[$','$]')
assignInNamespace(".units_options",opt,'units')
units_options('group')
` ``

## new

`r set_units(1,'m')`

`r format(set_units(1,'m'))`

`r format(set_units(1,'m*m'))`

`r format(set_units(1,'1/(m*m)'))`

This results in the following file

bla.pdf