Extend maxRechnung files

maxRechnung_ubl.xml and maxRechnung_creditnote.xml should contain as many elements as possible

remove unnecessary elements:

• UBL-CR-481 throws warning (done in !77 (merged))

add:

• multiple BG-20
• multiple BG-21
• BG-26 INVOICE LINE PERIOD (see !83 (merged))
• BG-DEX-05 SUB INVOICE LINE PERIOD (#124)
• BT-9 to maxRechnung_creditnote.xml
• BT-6 / BT-111 to both maxRechnung files (see #136 (closed))
• BT-146, BT-147, BT-148 with more than two digits (#139 (closed))
• multiple BG-32
• multiple BT-29 (see #143 (closed))
• multiple BT-158 (see #143 (closed))
• multiple BG-3 (see 04.03a)
• BG-24 alle BTs (124, 125, 125 mime code fehlen)
• multiple BG-17 (inkl. BT-85, BT-86)
• multiple BG-DEX-01
• multiple BG-DEX-03
• multiple BG-DEX-04
• multiple BG-DEX-08
• multiple BG-DEX-09