Rework transaction functions (Part 3)

Follows #72 (closed)

• allSources/all_input don’t need to be passed trough all f(). pubkey’s balance must be determined at the beginning, then passed trough all f().
• add f() to calculate amount in current base from the amount and the base of the source
• get_sources(): pending section:
  • usage of index with for for i, pending in enumerate(pendings):
  • move outside the for loop the condition (of bug). Is this check still relevant?
• merge intermediaries and final transactions (document generation, doc signing, post) into one f()