vsop: cleaning up subroutine vsop_lbr()
data: cleaned up subroutine readplanetdata()
Cleaning up VSOP code

* Cleaning up VSOP code:
  * in data.f90, subroutine readplanetdata():
    * define number of data lines here, rather than in vsop/vsop_lbr() upon every call

  * in vsop.f90, subroutine vsop_lbr():
    * compute LBR variables in a loop, rather than in blocks
    * do not copy the VSOP data for the given planet to a smaller local array
      * saves ~6% cpu time in gfortran
    * obtain pow and fac terms separately, for clarity and in preparation of accuracy
      determination for the current term
    * BUGFIX: last term of a L,B variable was used again as the first term of the next
      (B,R) variable.  Minor effect, since it was the last, tiny term.
vsop: determine accuracy reached after (almost) each term

* vsop: determine accuracy reached after (almost) each term:
  * estimate accuracy reached as specified by VSOP87 paper
    * every 3 iterations to reduce overhead
  * if reached, skip the remaining terms for the current power of tau
  * currently hardcoded to the truncation of VSOP87
    * saves ~15% CPU time without accuracy loss for the Sun in 2014

  * VSOP87 truncation and line numbers for powers of tau are set in
    readplanetdata() in data.f90