#133 add mil =1/1000 in and pt=1/72 in to bu_units_conversion

[SADOVSKI]

current version (7.30.10) of bu_units_conversion (in c and tcl) doesn't know about
pt = 1/72 in and mil = 1/1000 in
or one can even be more pedantic and distinguish
pt=1/72.27 in and bp = 1/72 in
along with more obscure things like
pc: pica (1 pica = 12 points)
see https://en.wikibooks.org/wiki/TeX/TeX_dimensions
this should be an easy fix: extend accordingly the table bu_units_length_tab[] in src/libbu/units.c