Review casts in PCL

Context

With the migration to index_t, PCL will soon have some unneeded casts which should be removed

Describe the solution you'd like

Each cast needs to be reviewed and removed if it's unneeded. A non-exhaustive list of such situations:

• assigning index_t to width
• assigning size_t to Indices

Additional context

This will require significant help from others since the scope is wide, but the issue is not hard at all. To get an idea of the scale, please check the occurrence of static_cast via:

grep '\s*static_cast' * -nrH | grep -v '<float>' | grep -v 'cast<\S*\*\s*>' | wc -l

Note: this doesn't count C style casts

(High effort for total change, low for effort needed for piece-meal contributions)