#128 Add const-poisoning

[Mikhail T.]

By explicitly declaring function-parameters const, one makes the code both easier for the compiler to optimize and for a human to read. The attached patch picks what seems to be the low-hanging fruit adding const-qualifiers to various "get" methods where immediately obvious. More of the same would be welcome.

[Hervé Drolon]

Hi,

This will need a change to the API/ABI and thus a brand new '4.0' release.
To do according to the available time.

Hervé