numapde-get-bib.sh introduces incorrect protection marks
`numapde-get-bib.sh 10.1007/978-3-030-50450-2_8` is OK but `numapde-get-bib.sh 10.1007/978-3-030-50450-2_8 --merge new.bib` introduces a stray backslash into the DOI:
```@INCOLLECTION{AlbiCristianiPareschiPeri:2020:1,
AUTHOR = {Albi, Giacomo and Cristiani, Emiliano and Pareschi, Lorenzo and Peri, Daniele},
PUBLISHER = {Springer International Publishing},
BOOKTITLE = {Crowd Dynamics, Volume 2},
DATE = {2020},
DOI = {10.1007/978-3-030-50450-2\_8},
PAGES = {159--197},
TITLE = {Mathematical Models and Methods for Crowd Dynamics Control},
}
```
A similar issues occurs with existing backslashes, as in `numapde-get-bib.sh 10.1137/17m1117872`, where the publisher entry contains `\\&` instead of the original and correct `\&`.
issue