Feature/allow allow any install folder

This pull request fixes the following issues:

install_heurist.sh creates some symbolic links (symlinks) which point on absolute paths.
This is all fine and dandy when installation folder is /var/www/html ; however, if the localhost folder is different, or in the context of Docker virtualization, these symlinks can become all fucked up.
Plus, absolute symlinks prevent moving the installation folder. Thus, relative paths should be used.
huma-num/heurist#1
install_heurist.sh hard-codes /var/www/html as the installation folder for Heurist. It's all over the place, really.
However, this is not always the case ; for example, Apache defaut localhost folder on MacOs is not /var/www/html, but /Library/WebServer/Documents instead.
To make installation on MacOS easier, this should be refactored.
As making the installation directory could break the existing CLI interface, it should at least be refactored into a variable to make changing it easier.
huma-num/heurist#2

These modifications of scripts install_heurist.sh and update_heurist.sh seem to work perfectly.
I didn't unit test these, 'cause it's always hard to test bash scripts stuffed with syscalls :cry:
Thus, given the critical nature of these two files, I suggest fully testing these modifications on your side. Don't trust me too much!

However, the current pull request does not allow running Heurist from any folder yet.
There seems to be more hard-coded /var/www elsewhere, or other direct or indirect assumptions about this installation path, in the codebase. I'll probably adress this later.

(This pull request stems from issues huma-num/heurist#1 and huma-num/heurist#2)

HeuristNetwork / heurist

Feature/allow allow any install folder #154