Rachel format nuclear data files

Rachel accepts the standard Gosia data in addition to the measured relative gamma-ray yield data, in the file <pre>rachel_nuclear_data.txt</pre>.

* Excited states' lifetimes
* Measured matrix elements (signed)
* Measured branching ratios (not including internal conversion branches)
* Measured E2/M1 mixing ratios  

The Rachel file format differs from the standard Gosia input for these data and is "human readable":

<pre>
branching initial_band initial_spin final_band_1 final_spin_1 final_band_2 final_spin_2 branching_ratio error
mixing    initial_band initial_spin final_band final_spin mixing_ratio error
lifetime band spin lifetime[ps] error[ps]
<final_band final_spin || multipolarity || initial_band initial_spin> = signed_matrix_element error
...</pre>

where "branching," "mixing," "lifetime" and the bra and ket notation are fixed formatting symbols and ''band'' is a valid text band name under Rachel.  Note the "upward" direction for the matrix element (from lower- to higher-numbered state) and the standard units of the appropriate powers of e, b and nuclear magnetons.  The individual entries can be given in any order, for example

<pre>
lifetime band1 2.5 27. 3.                            # in picoseconds
<band1 4.5 || E2 || band1 2.5> = 2.54 0.38           # Sign is interpreted as positive [eb]
mixing band1 2.5 band1 1.5 0.35 0.05                 # any user comment
<band5 5.5 || E3 || band1 2.5> = -0.23 0.12          # Sign is interpreted as negative [eb^(3/2)]
branching band2 2.5 band1 2.5 band1 1.5 0.100 0.020  # gamma decay branches only--not including conversion
lifetime band1 3.5 5.  2.                            # cite Jane Doe, PRC 5, 289 (1994)
</pre>

The order of the individual lines is completely arbitrary.  For example, see the example file [[File:Rachelnucleardata.txt]], which should be renamed as above so that rachel.py will find it in the working directory.