Rename attributes

I made the following renames:

* el_effective_mass -> electron_effective_mass
* ho_effective_mass -> hole_effective_mass
* accept_conc -> acceptor_concentration
* accept_ionization_energy -> acceptor_ionization_energy
Rename methods

I made the following renames:

* calc_cond_band_effective_dos -> calc_cb_effective_dos
* calc_val_band_effective_dos -> calc_vb_effective_dos
* calc_el_carrier_conc -> calc_electron_concentration
* calc_ho_carrier_conc -> calc_hole_concentration

Closes #42
Closes #44
Ignore conda virtualenv
Edit docstrings for clarity and consistency
Use sphinxy docstrings for 'symbol'

Closes #48.