Add lasversionconvert script

[kinverarity1]

This PR fixes #31 by implementing a command line script lasversionconvert:

C:\>lasversionconvert --help
usage: Convert LAS file version [-h] [-t TO] [--overwrite] [-i] input output

positional arguments:
  input
  output

optional arguments:
  -h, --help            show this help message and exit
  -t TO, --to TO        Version to convert to (default: 2)
  --overwrite           Overwrite output file if it already exists (default:
                        False)
  -i, --ignore-header-errors
                        Ignore header section errors. (default: False)

[dagrha]

Looks good to me!

Pip installed this branch to a fresh python 3.8.2 venv. Wish I had something to report but the script works flawlessly. 👍