Check exactextract library version on plugin init

[JakubCha]

Before initialization of the plugin, there is a check for the missing exactextract library. If there's no library it will be installed according to qgis-requirements.txt.
A case should also be handled for an old version of the exactextract library; in that case, a new version should be installed, otherwise new functionality won't work.

[JakubCha]

I am struggling with this. I would like to not add another python packages folder that would mess up things with the standard one - the worst case is exactextract in both places with different versions installed. It would happen in case the plugin installs the package into the custom, --user folder and then the user installs it by themself, using OSGEO4W shell.
Unfortunately, there's no standardized way to do that right now in QGIS. Consider releasing a plugin without this feature and letting the user install the library by themself.

qgis/QGIS-Enhancement-Proposals#202
qiceradar/qiceradar#42

[JakubCha]

#29 doesn't check for a package version but gives proper exception if package is outdated through the usage of new attributes (e.g. progress). It also gives information on how to update package.

[JakubCha]

New plugin version was released with changes introduced with #29