[TEP007] Isotope stratified file support

[vg3095]

This PR is related to adding support for reading stratified abundances with isotopes . This is rebased over PR #762 (Isotope uniform config)
For sample seeing output:
Notebook
abund.dat
Config

Just want to clear 2 things,

• I have assumed, only 30 normal elements (hardcoded in code), and rest isotopes.

• Currently the header is of following format (for Z=1 to 30 , numbers are used),
  1 2 3 4........30 Ni56 Ni58
  Do we need the format same as above or format below (represent Z=1 to 30 , as symbols):
  H He ......Zn Ni56 Ni58
  or you are thinking of some other format.

• Will number of normal elements be variable ? . If we want to make normal elements(Z<=30) variable in file, then I can think of this as of now,

  • if we follow 1st header format as I said above, then header is read as str format, because of mixed datatypes (1...30(int) then isotopes names(str) ), I could use regex here to find the index at which isotope names(str) starts,
  • if we follow 2nd header format, then I can use element_symbol2atomic_number function defined in util.py , and at the header index at which it raises MalformedElementSymbolError , isotopes will start.

[vg3095]

@unoebauer @yeganer Take a look at this PR, whenever you have time.