Update Linux installation notes

[jedgedrudd]

Describe the bug
INSTALL-LINUX.md: Following the Linux instructions on Ubuntu 18.04, dual-boot, fresh build: encountered some problems with running the SPM.py script and the SPMe.ipynb notebook.

1. line "sudo apt-get install python3 git-core python3-venv" could be extended to include python3-tk and graphviz.
2. lines "The safest way to install PyBaMM ....ready to exit the environment and go back to your original system, just type: deactivate" - add to this the following:
   a. whenever you close the terminal, or shut down, the environment is deactivated automatically.
   b. To go back into it, just run "source env/bin/activate" - this re-opens the same environment and the installs do not need to be rerun.
3. lines "...install both PyBaMM and its dependencies by typing: "pip install ." Or, if you want to install PyBaMM as a developer, use: "pip install -e .[dev,docs]"
   Make it clearer that this is a choice, just in case doing pip install . and then pip -e causes problems (probably doesn't).
4. Minor suggestion: add "Copy" buttons to each instruction: this can avoid typos and confusion.

To Reproduce
Install PyBaMM on a fresh Linux Ubuntu 18.04 installation.

*Expected behaviour
The instructions to run smoothly and the notebooks and scripts to work "out of the box".

Screenshots
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Additional context
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