Error when taking new_copy() of surface area parameters (lead acid)

[tobykirk]

Making new copies, using new_copy(), of some lead acid parameters (surface area to volume ratios) produces an error with print_name. @priyanshuone6

To Reproduce

import pybamm

param = pybamm.LeadAcidParameters()

x_n = pybamm.standard_spatial_vars.x_n
a_n = param.a_n(x_n)
a_n_new = a_n.new_copy()

Giving error:

Traceback (most recent call last):
  File "/home/user/PyBaMM/scripts-and-notebooks/FOQS_debugging.py", line 13, in <module>
    a_n_new = a_n.new_copy()
  File "/home/user/PyBaMM/pybamm/expression_tree/parameter.py", line 195, in new_copy
    self._input_names, self.orphans, print_name=self.print_name
  File "/home/user/PyBaMM/pybamm/expression_tree/parameter.py", line 224, in _function_parameter_new_copy
    print_name=print_name,
  File "/home/user/PyBaMM/pybamm/expression_tree/parameter.py", line 109, in __init__
    self.print_name = print_name
  File "/home/user/PyBaMM/pybamm/expression_tree/symbol.py", line 972, in print_name
    self._print_name = prettify_print_name(name)
  File "/home/user/PyBaMM/pybamm/expression_tree/printing/print_name.py", line 60, in prettify_print_name
    subscript_re = re.findall(r"^a_+(\w+)", name)[0]
IndexError: list index out of range

The same for the function parameter a_p and their dimensional versions a_n_dimensional, a_p_dimensional. Can make new copies just fine of any other parameter that I've tried though.

Additional context
Necessary for pull request #1529 where a size_average() is applied to interface variables in base_interface.py, which should do nothing for lead acid models, i.e. just make new copies of variables. The above error arises when building the FOQS lead acid model.